Effects of Immobilization Devices on Skin Doses and Doses in

UNIVERSITY OF GHANA

COLLEGE OF BASIC AND APPLIED SCIENCES

EFFECTS OF IMMOBILIZATION DEVICES ON SKIN DOSES AND DOSES IN

THE BUILD-UP REGIONS FOR HIGH ENERGY PHOTON BEAMS

BY

BISMARK DJAN

(10598536)

THIS THESIS/DISSERTATION IS SUBMITTED TO THE UNIVERSITY OF

GHANA, LEGON IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR

THE AWARD OF MPHIL MEDICAL PHYSICS DEGREE

JULY, 2018

University of Ghana http://ugspace.ug.edu.gh

ii

DECLARATION

This thesis is the result of a research work undertaken by Bismark Djan in the

Department of Medical Physics, School of Nuclear and Allied Sciences, University of

Ghana – Legon. The research was done under the able supervision of Prof. J. H. Amuasi,

Dr. Francis Hasford, and Mr. George Felix Acquah all from the Department of Medical

Physics.

It is my conviction that, no part of this research work has been presented in part or whole

to any other institution or university for the award of a diploma, or degree at any level.

Accordingly, other works and/or researches done by other researchers cited in this work

have been duly acknowledged under references.

…………………………….. Date …………………… Bismark Djan (Student’s ID: 10598536) …………………………….. Date …………………… Dr. Francis Hasford (Principal Supervisor) …………………………….. Date …………………… Prof. J. H. Amuasi (Co-Supervisor) …………………………….. Date …………………… Mr. George Felix Acquah (Co-Supervisor)


iii

ABSTRACT

This study presents the significant increase in skin dose caused by the presence of

thermoplastic immobilizing masks used for positioning and immobilization of patients

during cancer treatments in the head and neck region. Made from organic materials,

thermoplastics become soft and pliable when heated and forms a rigid replication of the

patient’s anatomy as it cools down. The use of the mask plays a key role in achieving the

therapeutic aim in head and neck treatments but can as well lead to a rise in surface dose.

The purpose of this research is to examine and analyze the effects of thermoplastic masks

used as immobilization devices on skin and the build-up region doses for a 6 MV Three

Dimensional Conformal Radiotherapy plan. Measurements in the solid water phantom

coupled with an electrometer and a Roos type ionization chamber at the surface (skin),

the dmax (1.6 cm) and a specified depth of 10 cm of the 6 MV photon beam were made

and recorded employing the isocentric technique for known varying field sizes with 100

MU. Measurements with or without the mask were also made on the phantom.

GafChromic films were also used to assess the skin dose and dmax and compared to the

measured values. Surface dose increased significantly due to the presence of the

thermoplastic masks. It was observed that the estimated surface doses with the presence

of the thermoplastic mask were consistently larger than those without the mask. Due to

the presence of the thermoplastic mask during measurements, the dose in the build-up

region increased, subsequently shifting the depth of maximum dose, dmax, to shallower

depths to the skin surface. For a field size of 10 x 10 cm2, an increase in skin dose of

0.21, 0.13 and 0.05 Gy was estimated for the unstretched mask, the 5 cm stretched mask

and the 10 cm stretched mask respectively, which was ascribed to the “bolus” effect of


iv

the thermoplastic masks. For the various field sizes, the skin dose increased by an

average factor less than 1% for the different mask stretches. However, a variation within

1% was recorded although the skin dose decreased as the mask was extended. The

thermoplastic factor at the skin surface was also found to increase with an increasing

mask thickness and for a particular mask type (unstretched, 5 cm stretched, or 10 cm

stretched) the thermoplastic factor decreased with increasing field size. The discrepancies

obtained using the GafChromic films varied significantly with those obtained using the

ionization chamber which was associated with the heterogeneous composition of the film

arising from the manufacturing process. The skin-sparing effect which is an advantage

for megavoltage beams was not achieved due to the presence of the masks.


v

DEDICATION

This thesis work is dedicated to God Almighty for love, care, provisions,

encouragements, strength, protection, and blessings He has given me throughout my

education and above all, for the wisdom, He has been granting me since secondary school

till now. Also, I dedicate this work to my irreplaceable family for their love, care,

encouragements, support and prayers throughout my education and most especially to my

sister Mrs. Esther Anokye.


vi

ACKNOWLEDGMENTS

My first and foremost thanks and appreciation go to God Almighty for protecting and

guiding me, giving me the sound health, wisdom, knowledge and providing me with all

that I needed throughout this research work. May His sovereign name be praised now and

forever more.

I am very much grateful to the International Atomic Energy Agency for making it

possible for me to undertake this Master of Philosophy Degree programme in Medical

Physics here in Ghana.

I sincerely salute my able supervisors, Prof. J.H. Amuasi, Dr. Francis Hasford, and Mr.

George Felix Acquah for their continued pieces of advice, corrections, directions,

comfort, sacrifices and above all the love and commitments they had in helping me

through with my work. No word can express my joy at having you as my team of

supervisors, I truly appreciate your support.

Also, to my parents and siblings, I am very much grateful for all the support, advice,

encouragements and love you gave me during my entire life in school. I pray for Gods

blessings for you all.

Lastly, my special thanks go to my Head of Department, Dr. Francis Hasford who is also

my principal supervisor and all the Lecturers for their continued encouragements and

motivations.


vii

TABLE OF CONTENTS

DECLARATION............................................................................................................... ii ABSTRACT ...................................................................................................................... iii

DEDICATION....................................................................................................................v

ACKNOWLEDGMENTS ............................................................................................... vi

TABLE OF CONTENTS ............................................................................................... vii LIST OF FIGURES ...........................................................................................................x

LIST OF TABLES .......................................................................................................... xii

LIST OF ABBREVIATIONS ....................................................................................... xiii

CHAPTER ONE ................................................................................................................1

INTRODUCTION..............................................................................................................1

1.1 Background ....................................................................................................... 1

1.2 Problem Statement............................................................................................ 6

1.3 Objectives........................................................................................................... 7

1.4 Relevance and Justification .............................................................................. 8

1.5 Scope and Limitation ........................................................................................ 9

1.6 Organization of Thesis ...................................................................................... 9

CHAPTER TWO .............................................................................................................10

LITERATURE REVIEW ...............................................................................................10

2.1 Introduction ..................................................................................................... 10

2.2 Skin and Buildup Region Doses ..................................................................... 11

2.2.1 Electron Contamination of Photon Beams ............................................. 12

2.2.2 Skin-Sparing Effect as A Function of Photon Energy ........................... 13

2.2.3 Effects of Thermoplastic Immobilizers on Surface Dose. ..................... 15

2.2.4 Effect of Field Size on Skin/Surface Dose ............................................... 18

2.3 Exit Dose .......................................................................................................... 19

2.4 Radiation Dosimeters for Skin Dose Measurements ................................... 20

2.4.1 Ionization Chambers ................................................................................ 20

2.4.2 Electrometers............................................................................................. 21

2.4.3 Radiochromic Film ................................................................................... 22

CHAPTER THREE .........................................................................................................27

MATERIALS AND METHODS ....................................................................................27


viii

3.1 Materials .......................................................................................................... 27

3.1.1 Medical Linear Accelerator ..................................................................... 27

3.1.2 Ionization Chamber .................................................................................. 29

3.1.3 Electrometer .............................................................................................. 31

3.1.4 Diodes ......................................................................................................... 32

3.1.5 Water Bath ................................................................................................ 33

3.1.5 Thermoplastic Immobilization Mask ...................................................... 33

3.1.6 GafChromic Films .................................................................................... 34

3.1.7 ImageJ Software........................................................................................ 34

3.1.8 Other Materials ......................................................................................... 35

3.2 Experimental Method ..................................................................................... 36

3.2.1 Part 1: ......................................................................................................... 36

3.2.2 Part 2: ......................................................................................................... 40

3.2.3 Part 3: ......................................................................................................... 44

CHAPTER FOUR ............................................................................................................46

RESULTS AND DISCUSSION ......................................................................................46

4.1 Introduction ..................................................................................................... 46

4.2 Dose Determination from Ion chamber ........................................................ 46

4.2.1 Graphical Representations of Absorbed Dose against Depth ............... 50

4.3 Percentage Depth Dose (PDD) Determination ............................................. 52

4.3.1 Variation of Percentage Depth Dose (PDD) with Field Size and Depth. 54

4.4 Determination of Skin Dose from PDD values ............................................. 57

4.4.1 Effect of Field Size on Skin Dose. ............................................................ 60

4.4.2 Verification of Prescribed Dose ............................................................... 60

4.5 Thermoplastic Factors, TF ............................................................................. 61

4.6 Calibration Curve ........................................................................................... 62

4.6.1 Dose Determination for the Irradiated GafChromic EBT3 Films. ...... 65

CHAPTER FIVE .............................................................................................................73

CONCLUSION AND RECOMMENDATIONS ...........................................................73

5.1 Conclusions ...................................................................................................... 73

5.2 Recommendations ........................................................................................... 75

5.3 Further Research ............................................................................................ 75


ix

REFERENCES .................................................................................................................76

APPENDICES ..................................................................................................................82

APPENDIX 1 ............................................................................................................... 82

APPENDIX 2 ............................................................................................................... 82

APPENDIX 3 ............................................................................................................... 84

APPENDIX 4 ............................................................................................................... 86

APPENDIX 5 ............................................................................................................... 86

APPENDIX 6 ............................................................................................................... 88

APPENDIX 7 ............................................................................................................... 89

APPENDIX 8 ............................................................................................................... 90

APPENDIX 9 ............................................................................................................... 91

APPENDIX 10 ............................................................................................................. 92

APPENDIX 11 ............................................................................................................. 93


x

LIST OF FIGURES

Figure 1. 1: An illustration of the concept of therapeutic ratio. Image from (“Basics of Radiation Therapy | Clinical Gate,” 2015)...........................................................................2

Figure 1. 2: Physical evidence of the effects of a thermoplastic mask on patient skin showing broken skin after five weeks of radiation treatment. Image obtained from (Thomas, n.d.) ......................................................................................................................5

Figure 2. 1: A graph of absorbed dose and kerma as functions of depth ...........................12

Figure 2. 2: An unstretched thermoplastic mask ...............................................................17

Figure 2. 3: A stretched thermoplastic mask to be used on the solid water phantom ........17

Figure 2. 4: A thermoplastic mask fixed to the couch in the treatment room....................17

Figure 2. 5: A moulded thermoplastic mask during simulation.........................................17

Figure 2. 6: The effect of thermoplastic mask on skin dose ..............................................17

Figure 2. 7: Percentage skin dose against field size. (Data from Velkley, et al., 10 MV data are from Khan FM, et al.) ...........................................................................................19

Figure 2. 8: Structure of GafChromic EBT3 Dosimetry Film (Actual thickness may vary slightly). .............................................................................................................................24

Figure 3. 1: Medical Linear Accelerator at Sweden Ghana Medical Centre (SGMC) ......29

Figure 3. 2: Roos Chamber type 34001 .............................................................................30

Figure 3. 3: Inserting/removing the Roos Chamber...........................................................31

Figure 3. 4: Twin plate with inserted Roos chamber .........................................................31

Figure 3. 5: PTW UNIDOS Electrometer ..........................................................................32

Figure 3. 6: Diode ..............................................................................................................32

Figure 3. 7: Water Bath ......................................................................................................33

Figure 3. 8: Image J software interface ..............................................................................35

Figure 3. 9: Barometer .......................................................................................................35

Figure 3. 10: Diode reader .................................................................................................35

Figure 3. 11: Solid water phantom .....................................................................................35

Figure 3. 12: Thermometer ................................................................................................35

Figure 3. 13: Setup without a thermoplastic mask .............................................................37

Figure 3. 14: Setup with an unstretched thermoplastic mask ............................................37

Figure 3. 15: Setup with a 5 cm stretched thermoplastic mask .........................................38

Figure 3. 16: Positioning of the film set A, B, and C at the surface of the phantom, at the dmax of 1.6 cm for the 6 MV photon beam, and at a depth of 10 cm respectively .............41

Figure 3. 17: Film placement on the scanner .....................................................................43

Figure 3. 18: ImageJ software interface with a scanned film image split into the RGB channels..............................................................................................................................44

Figure 3. 19: Treatment planning process for the dose verification ..................................45

Figure 4. 1: A graphical representation of Absorbed dose against Depth for no thermoplastic mask. .........................................................................................................50


file:///C:/Users/MARK/Documents/Bismark%20Djan%20Final%20Thesis%20for%20printing%20with%20Corrections%20Effected.doc%23_Toc527579795











xi

Figure 4. 2: A graphical representation of Absorbed dose against Depth for the unstretched thermoplastic mask. ....................................................................................51

Figure 4. 3: A graphical representation of Absorbed dose against Depth for the 5 cm stretched thermoplastic mask. ........................................................................................51

Figure 4. 4: A graphical representation of Absorbed dose against Depth for the 10 cm stretched thermoplastic mask. ........................................................................................52

Figure 4. 5: A graph of PDD against Depth for a 10x10 field size with an unstretched mask and without a mask. ..................................................................................................53

Figure 4. 6: A graph of PDD against Depth for a 10x10 field size with a 5 cm stretched mask and without a mask. ..................................................................................................53

Figure 4. 7: A graph of PDD against Depth for a 10 x 10 field size with a 10 cm stretched mask and without a mask. ..................................................................................................54

Figure 4. 8: A graph showing the variation of PDD versus depth and field size for measurements without the thermoplastic mask..............................................................55

Figure 4. 9: A graph showing the variation of PDD versus depth and field size for measurements with an unstretched thermoplastic mask. ...............................................56

Figure 4. 10: A graph showing the variation of PDD versus depth and field size for measurements with a 5 cm stretched thermoplastic mask. ............................................56

Figure 4. 11: A graph showing the variation of PDD versus depth and field size for measurements with a 10 cm stretched thermoplastic mask. ..........................................57

Figure 4. 12: A bar chart showing the effect of different mask thickness on PDD at depth 0.0 cm. ................................................................................................................................58

Figure 4. 13: A bar chart showing the effect of different mask thickness on the skin dose at depth 0.0 cm. ..................................................................................................................59

Figure 4. 14: Calibration curve comparing RGB channels for 6 MV photon energy. .......63

Figure 4. 15: A graph of Absorbed Dose against Net Optical Density for the Red channel.64

Figure 4. 16: A graph showing the variation of absorbed dose versus depth and field size for GafChromic measurements without the mask. ..........................................................70

Figure 4. 17: A graph showing the variation of absorbed dose versus depth and field size for GafChromic measurements with the unstretched mask. ............................................71

Figure 4. 18: A graph showing the variation of absorbed dose versus depth and field size for GafChromic measurements with the 5 cm stretched mask. .......................................71

Figure 4. 19: A graph showing the variation of absorbed dose versus depth and field size for GafChromic measurements with the 10 cm stretched mask. .....................................72


xii

LIST OF TABLES

Table 2. 1: Buildup Dose Distribution in Polystyrene for a 10 × 10 cm2 Field .................14

Table 4. 1: Measured Absorbed dose (Gy) values for various field sizes at different depths without the mask. .................................................................................................47

Table 4. 2: Measured Absorbed dose (Gy) values for various field sizes at different depths with the unstretched mask. ...................................................................................47

Table 4. 3: Measured Absorbed dose (Gy) values for various field sizes at different depths with the 5 cm stretched mask. ..............................................................................48

Table 4. 4: Measured Absorbed dose (Gy) values for various field sizes at different depths with the 10 cm stretched mask .............................................................................49

Table 4. 5: Dose (Gy) differences between the various mask stretches and without the mask ...................................................................................................................................59

Table 4. 6: Skin Doses (Gy) at a depth of 0.0 cm for the various Stretches of the thermoplastic mask. ...........................................................................................................60

Table 4. 7: Thermoplastic factor for the unstretched thermoplastic mask. ...................61

Table 4. 8: Thermoplastic factor for the 5 cm stretched thermoplastic mask. ...............62

Table 4. 9: Thermoplastic factor for the 10 cm stretched thermoplastic mask. .............62

Table 4. 10: Net Optical Densities as a function of absorbed dose for the calibrated GafChromic films ..............................................................................................................63

Table 4. 11: Net Optical Densities (Net OD) obtained for the GafChromic films without the mask. ...........................................................................................................................65

Table 4. 12: Net Optical Densities (Net OD) obtained for the GafChromic films with the unstretched mask. ............................................................................................................65

Table 4. 13: Net Optical Densities (Net OD) obtained for the GafChromic films with the 5 cm stretched mask.........................................................................................................65

Table 4. 14: Net Optical Densities (Net OD) obtained for the GafChromic films with the 10 cm stretched mask.......................................................................................................66

Table 4. 15: Calculated absorbed dose (Gy) values for various field sizes at different depths without the mask. .................................................................................................66

Table 4. 16: Calculated absorbed dose (Gy) values for various field sizes at different depths with the unstretched mask. ...................................................................................67

Table 4. 17: Calculated absorbed dose (Gy) values for various field sizes at different depths with the 5 cm stretched mask. ..............................................................................68

Table 4. 18: Calculated absorbed dose (Gy) values for various field sizes at different depths with the 10 cm stretched mask. ............................................................................69


xiii

LIST OF ABBREVIATIONS

SGMC Sweden Ghana Medical Centre

MPHIL Master of Philosophy

ICRU International Commission on Radiation Units and Measurements

MU Monitor Units

3DCRT Three-Dimensional Conformal Radiotherapy

TG Task Group

TPS Treatment Planning System

SAD Source to Axis Distance

CT Computed Tomography

SSD Source to Surface Distances

PDD Percentage Depth Dose

IMRT Intensity-Modulated Radiation Therapy

ICRP International Commission on Radiological Protection

MV Mega Voltage

AAPM American Association of Physicist in Medicine

CPE Charge Particle Equilibrium

Dmax Depth of Maximum Dose

LINACs Linear Accelerators

TCP Tumor Control Probability

IEC International Electrotechnical Commission

NTCP Normal Tissue Complication Probability


xiv

TIFF Tagged Image File Format

TLD Thermoluminescent dosimeter

RGB Red, Green and Blue

EF-IMRT Extended Field Intensity-Modulated Radiotherapy

TMR Tissue Maximum Ratio

OD Optical Density

PMMA Polymethylmethacrylate

UV Ultra Violet

PTV Planning Target Volume

IAEA International Atomic Energy Agency

CRT Conformal Radiation Therapy

Co-60 Cobalt-60 Teletherapy unit

QC Quality Control

TF Thermoplastic Factor

EBTXD EBT-eXtended Dose


1

CHAPTER ONE

INTRODUCTION

1.1 Background

Ever since Wilhelm Conrad Roentgen begun with the discovery of X-rays on the 8th

November 1895 at the University of Wurzburg and Antoine Henri Becquerel coined the

radioactivity phenomenon in 1896, cancer treatments have been appreciated through the

use of ionizing radiation. (Podgorsak & Rosenberg, 2008) stated that “radiotherapy, also

known as radiation therapy, radiation oncology or therapeutic radiology, is one of the

three major modalities used in the treatment of cancers, where electromagnetic and

particle radiations are used, with the other two being surgery and chemotherapy”.

Radiation therapy subsequently has gained recognition and has tremendously improved

and developed into one of the important specialized medical fields. (Podgorsak &

Rosenberg, 2008) have also stated that “X-rays are mostly used in diagnostic radiology

for diagnosing diseases and in radiation oncology for the treatment of cancerous tumors”.

Radiation therapy is generally characterized by two major forms considering the distance

between the radiation source and the tumor, to be treated. Brachytherapy “is a method of

treatment in which sealed radioactive sources are used to deliver radiation at a short

distance by interstitial, intracavitary, or surface application” as stated by (Khan, 2014). In

this method of therapy, the high radiation dose is delivered locally to the tumor with rapid

dose fall-off in the surrounding normal tissue. The second form of radiation therapy is

known as external beam radiotherapy. (Podgorsak & Rosenberg, 2008) describes this as a

type of radiation therapy which has the radiation source at a distance away from the

patient and the tumor target within the patient is irradiated with an external radiation


2

beam. Most of the external beam radiation therapy is performed with photon beams,

some with electron beams and a very small fraction with more exotic particles such as

heavy ions, protons or neutrons.

Studies by (Sweeney et al., 1998) have shown that the accurate delivery of a prescribed

radiation dose to a target tumor while sparing surrounding critical and normal tissues, is

the key objective of radiotherapy. The International Commission on Radiation Units and

Measurements (ICRU) recommends that to obtain this accuracy, it is required that the

error in total dose be within ±5% (Musolino, 2000). Figure 1 illustrates the concept of

therapeutic ratio explained by (Williams & Thwaites, 2000) as delivering the highest

probable dose to the tumor and thus the maximum probability of complete tumor

regression.

Figure 1. 1: An illustration of the concept of therapeutic ratio. Image from (“Basics of Radiation Therapy |

Clinical Gate,” 2015)

Radiotherapeutic success is most probable when the two curves in Figure 1 are widely

separated (Williams & Thwaites, 2000). The further the normal tissue complication


3

probability (NTCP) curve is to the right of the tumor control probability curve (TCP), the

greater the therapeutic ratio and the lesser the treatment complications.

The prescribed treatment can either be for curative or palliative intent. In the case of

curative purpose, the side effects are sometimes unavoidable, but they are accepted as an

inevitable part of the cure. On the other hand, (Tabakov et al., 2008) stated that “where

cure from radical treatment is unlikely, palliative treatment can alleviate painful or

distressing symptoms and restore a higher degree of life quality for patients”. A target

dose uniformity within +7% and -5% is recommended by the ICRU Report number 50

for the radiation dose delivered to a precise prescription point within the target (Jones,

1994).

In external beam radiation therapy, medical linear accelerators and Cobalt-60 units are

used for cancer treatments and the procedures of treatment commonly rely on different

conditions such as the;

Shape, size, and position of the cancerous cells.

Location of sensitive or critical organs within the locality of the target volume

(that is, sparing of normal tissues) and

Financial constraints and the quest for optimization of radiation dose to the target

volume.

(Khan, 2014) indicates that “skin-sparing effect is one of the most desired features of

megavoltage beams, and all effort should be focused toward preserving this effect when

irradiating normal skin”. However, this effect may be reduced or even lost if the radiation

beam is extremely contaminated with secondary electrons. Consequently, skin dose

should be negligible during this treatment delivery, but this is never achieved because it


4

depends on secondary electrons (Yadav et al. 2009). These secondary electrons are

mostly produced by the interaction of photons with some scattering materials such as

collimator jaws, air, patient's skin and beam modifiers. (Nilsson & Brahme, 1986) has

shown in their studies that the air column under block tray has a more substantial

contribution at lower energies.

(Prabhu, 2012) has shown that positioning and immobilization of patients are the most

crucial parts of radiotherapy treatment. Therefore, in order to achieve this therapeutic aim

in the head and neck areas of cancer treatment, patients undertaking radiation therapy

must be well immobilized. Without proper immobilization such as the use of

thermoplastic masks, a patient is probably at risk for wrong or improper treatment and

this can result in unwanted side effects. In his study, (Meyer, 2014) stated that “a skin

condition called radiation dermatitis is one of the most common side effects of radiation

treatment and can range from a mild, red rash (erythema) and itchy, peeling or flaking

skin to a more severe reaction with blisters and wet, peeling skin”.

Thermoplastic is a plastic material (polymer) which turn out to be pliable or mouldable

above a certain temperature and hardens upon cooling. The main goal of a thermoplastic

mask is to provide a dependable method for accurate patient positioning and

immobilization during radiation head and neck treatments as well as patient repositioning

from one treatment to another and improving patient comfort. The thermoplastic mask is

first placed in warm water at a specified temperature to soften it and make it pliable, then

pulled over the patient’s face and moulded to the contours of his/her anatomy. As the

thermoplastic mask cools down, a firm replication of the patient’s anatomy is then

produced, and the thermoplastic mask is then fastened directly to the treatment couch.


5

Head and neck cancer treatments require a high degree of accuracy and the use of these

thermoplastic masks provides the required accuracy. It is, therefore, important to

determine its effect on patient’s skin and build-up region doses. Figure 2 shows the effect

of thermoplastic masks on a patient’s skin after five weeks of head and neck cancer

treatment.

Figure 1. 2: Physical evidence of the effects of a thermoplastic mask on patient skin showing broken skin after five weeks of radiation treatment. Image obtained from (Thomas, n.d.)

(Hsu et al., 2008), in their study, indicated that accurate evaluations of surface and

superficial doses in radiation therapy can present keen information for clinical

deliberation and at the same time controlling severe skin toxicity, particularly for breast

and head-and-neck treatments since dose at the skin is primarily as a result of electron

contamination arising from the flattening filter, beam modifiers, and air.

This study is aimed at assessing the dosimetric effects caused by thermoplastic

immobilizing devices on skin and build-up region doses for a 6 MV photon beam using a

Roos Chamber type 34001 coupled with UNIDOS Electrometer and an EBT3

GafChromic film for different field sizes for patients undergoing cancer treatments in the


6

head and neck using a medical linear accelerator at the Sweden Ghana Medical Center in

Accra.

1.2 Problem Statement

In radiation therapy, there is the possibility of encountering several problems since

tumors in areas like head and neck are in close proximity to healthy tissues/critical organs

and the radiation has to travel through these healthy tissues before reaching the target

volume. Consequently, patients are required to be properly immobilized using

immobilization devices to provide precision/accuracy during treatments, enhance

reproducibility from one treatment fraction to another, to reduce daily setup errors and

also increase patient comfort.

In view of the proximity of the tumor to the surrounding critical organs and normal

tissues with comparatively limited intra-fractional motion in head and neck cancers,

proper immobilization technique will allow smaller treatment margins and reduce the

side effects.

However, recent works have shown that the use of these immobilization devices

(thermoplastic mask) tends to attenuate part of the radiation beam and as a result,

increasing skin toxicity and the dose to the build-up region. From literature, the presence

of the thermoplastic mask tends to have some effects on the dosimetric outcomes (skin or

surface dose, dose at maximum depth, and at a depth of d = 10 cm). Because of the

uneven stretching of the heated thermoplastic mask, one ends up having a mask with

varying holes which also contributes to dosimetric effects on the patient. This can,


7

however, have an effect on the quality of life of the patients after treatment. Since these

effects are mostly ignored in practice, it is, therefore, important to analyze their impacts

on radiation therapy for head and neck cancer treatments.

This study thus seeks to investigate the operational challenges of using thermoplastic

immobilization mask and find out how its dosimetric effects can be reduced.

1.3 Objectives

The main aim of this study is to examine and analyze the effects of thermoplastic masks

used as immobilizing devices on the skin and the build-up region doses. The study

involves 3D conformal radiotherapy (CRT) plan for patients receiving head and neck

cancer treatments using 6 MV external radiation therapy (medical linear accelerator) with

a flat ionization chamber and EBT3 GafChromic films dosimetry.

The specific objectives are to:

1. Determine the LINAC’s output factors.

2. Determine the thermoplastic percentage depth dose and thermoplastic factor.

3. Determine the skin dose and the dose at maximum depth (Dmax), and also the

effects of the thermoplastic mask on them.

4. Use an anthropomorphic phantom for dose verification.

5. Make appropriate recommendations from the findings.


8

1.4 Relevance and Justification

Since the primary aim of radiotherapy is to accurately and precisely deliver a prescribed

dose to a target volume while sparing surrounding healthy and critical tissues, the

dosimetric effect of certain treatment parameters must not be overlooked in today’s

radiation therapy.

In radiotherapy, the accurate and precise delivery of radiation doses critically depends on

the day-to-day reproducibility of patient positioning. Now, different immobilization

devices are available to help in providing precise repositioning during radiation

treatments and also increasing patient comfort. Thermoplastic masks are widely used in

head and neck cancer treatments because cancer in the head and neck areas are in close

proximity to critical organs and it is required that patients are properly immobilized in

order to achieve the goals of immobilization devices.

During simulations or treatments, lasers are used in aligning the patient in the room and

these thermoplastic masks provide suitable space for the use of fiducial markers which

eliminates the markings on the skin (face) of the patient. On the contrary, the presence of

these immobilizing devices (thermoplastic masks) have the tendency of attenuating part

of the radiation beam and further increasing skin and build-up region doses which could

change the intended plan. For that reason, understanding how these devices affect skin

and the build-up region doses in a particular treatment form is of great importance. This

would ensure that medical physicists consider their effects and hence make appropriate

setup corrections and other adjustments (that is, firmly stretching the mask to reduce its

thickness) to keep acceptable doses to skin and build-up regions.


9

1.5 Scope and Limitation

Some works have been done to investigate skin and build-up region doses in radiotherapy

using various methods. However, this study focuses on employing the application of

thermoplastic masks as the immobilization device stretched at three different lengths in

order to obtain different hole diameters (unstretched, 5 cm stretched, and 10 cm

stretched) during head and neck cancer treatments with a 6 MV 3D conformal Plan in

radiation therapy. This was done at the Sweden Ghana Medical Centre using the medical

linear accelerator. The Roos Chamber type 34001 and the GafChromic EBT3 films are

used in this study because they are user-friendly and can easily be used to measure the

surface dose whiles on the other hand, the Farmer type Ion Chamber cannot be used to

measure the surface dose directly but can only be determined from the PDD.

Measurements are made in a solid water phantom at the surface (skin), the build-up

region, the dmax = 1.6 cm and a specified depth of 10 cm for the 6 MV photon beam at

field sizes of 5 x 5, 10 x 10, 15 x 15, and 20 x 20 cm2.

1.6 Organization of Thesis

This study is arranged in a chronological order of five chapters. Chapter One introduces

the research of which comprises; the background of the study, problem statement,

objectives, relevance, and justification, scope and limitation. Chapter Two reviews

existing literature relevant to the research problem. Chapter Three focuses on the

materials and methodologies used for the study. Chapter Four presents the results

obtained and discussions performed. Chapter Five is the conclusion of the study and

recommendations made from the study.


10

CHAPTER TWO

LITERATURE REVIEW

2.1 Introduction

Surface dose varies quite significantly using megavoltage radiations in clinical

radiotherapy. These differences can be ascribed to variations in electron contamination

and phantom scatter (Alnawaf et al., 2012). Majority of these discrepancies are detected

for fluctuations in some beam parameters such as use of beam modifying devices, field

size and the radiation beam angle of incidence (Hounsell & Wilkinson, 1999; M. J.

Butson et al., 1996) or due to the composition of the material which may be in the direct

or indirect path of the radiation beam in question (Martin J. Butson et al., 2007).

During external beam radiation therapy, the skin is mostly in danger from radiation

effects such as skin shedding, necrosis, and erythema. Epidemiological studies have also

found a link between basal cell carcinoma and radiotherapy (Kry et al., 2012). However,

skin dose is complicated because of the different skin layers with various depths of

thickness that varies between patients and positions on the patient. Besides, the

International Commission on Radiological Protection (ICRP) recommends measuring the

radiation dose to the skin at 0.07 mm (ICRP, 1977; ICRU, 1985) and 1.0 mm (M. Butson

et al., 1997) depth for the basal layer which is taken as the surface dose and dermal layer

respectively. (Court et al., 2008); (Dogan & Glasgow, 2003); (Panettieri et al., 2009)

have indicated that even though the surface dose may be evaluated with the TPS, mostly

within ± 25% accuracy, this method requires computed tomography images and an

estimated treatment plan. (Lamb & Blake, 1998) in their studies proposed a skin dose

estimation technique on the beams central axis as a function of the treatment parameters.


11

Nevertheless, their model evaluated only 6 MV and 10 MV photon beams and it is

limited to the central axis of the radiation beam.

2.2 Skin and Buildup Region Doses

When a patient is undergoing radiation therapy with a high energy photon, the surface

dose is said to be significantly lower than the radiation dose at the dmax that occurs in the

subcutaneous tissues. Notwithstanding the rise in extreme ionization at or close to the

skin as a result of the lower-energy beams, the megavoltage or high energy photon beams

produce an initial electronic build-up with depth. Consequently, the skin dose is less than

that at dmax which occurs at a depth downstream. Research has shown that the higher the

radiation energy, the deeper the penetration power and the dmax. (F. M. Khan, 2014)

stated that “for a higher energy photon beams, dose in the build-up region results from

the relatively long range of energetic secondary charged particles (positive and negative)

that are first released in the patient by photon interactions and then deposit their kinetic

energy in the patient”. The low skin dose is known as the skin-sparing effect, which is

mostly absent in superficial or orthovoltage beams. This effect, which is a property of

megavoltage beams may be reduced or even lost if the radiation beam is extremely

contaminated with secondary electrons. Orthovoltage and superficial beams do not

demonstration skin-sparing effects given that the maximum dose transpires at the

patient’s skin surface. Immediately under the patient’s skin surface, the condition of

charged particle equilibrium (CPE) ceases to exist and the absorbed radiation dose is thus

much smaller than the collisional kerma. However, with increasing depth in the patient,

the charged particle equilibrium is finally reached at d = dmax where d is nearly equal to


12

the range of the secondary charged particles and the radiation dose becomes comparable

with the collisional kerma. Beyond d = dmax, both collisional kerma and absorbed dose

decreases due to photon attenuations in the patient that results in transient rather than true

CPE (Podgorsak & Rosenberg, 2005).

2.2.1 Electron Contamination of Photon Beams

The surface dose in megavoltage photon beams is predominantly due to the

contamination of electrons in the incident radiation beam in conjunction with the

backscattered ionizing radiation (both electrons and photons) from the material medium.

Figure 2. 1: A graph of absorbed dose and kerma as functions of depth

(Biggs & Ling, 1979); (Padikal & Deye, 1978) indicated that electrons were the cause of

the increased dose. On the other hand, other researchers in (Biggs & Ling, 1979) iterated

that for the Clinac-35 linear accelerator (Varian Corporation, Palo Alto, California), the

additional dose in the build-up region is as a consequence of electrons, photons or both.

However, no attempts were made to clarify the relative importance of each entity. The

depth dose characteristics of a 34 MV photon beam from an Asklepitron 45 (a 45 MeV


13

betatron) was studied by Dawson (Biggs & Ling, 1979) and drew a conclusion that

electrons were the greatest contributor. Also, (Marbach & Almond, 1977) in their

publication postulated that for a 25 MV radiation beam from a Sagittaire, Compton

scattered photons were the cause for the added dose in the build-up region. Studies have

shown that virtually all X-ray and gamma-ray beams employed in radiotherapy mostly

face contamination with secondary electrons (mostly Compton electrons) produced by the

photon interactions with the collimator, air, the flattening filter, and other materials in the

path of the beam such as wedges and shadow tray. Electron contamination also disturbs

the disparity of dose in the build-up region with field size. (Dogan & Glasgow 2003)

described a decreased build-up region dose when comparing IMRT strip fields to open

beam fields and established that any rise in skin toxicity was not inherently due to IMRT

delivery techniques. (Biggs & Ling, 1979); (F. M. Khan, 2014) stated that “as the field

size increases, the depth dose in the build-up region increases, resulting in a shift in the

depth of maximum dose, dmax, to increasingly shallower depths”. Researchers such as

(Biggs & Ling, 1979); (Marbach & Almond, 1977); (Padikal & Deye, 1978) have shown

that this effect is mainly triggered by the secondary electrons.

2.2.2 Skin-Sparing Effect as A Function of Photon Energy

Researchers have indicated that the dose distribution in the build-up region relies on

numerous variables such as SSD, field size, photon beam energy, and configuration of the

secondary blocking trays (F. M. Khan, 1971; Pillai et al., 1973; Velkley et al., 1975).

Certain studies have indicated that there is a reduction in skin dose at higher photon beam

energies (Martin J. Butson et al., 1998; Chiu-Tsao & Chan, 2009; Lin et al., 2001;


14

Stathakis et al., 2006). However, other studies provide contrary view, particularly for

large field sizes (M. Butson et al., 1997; De Ost et al., 1997; Kim et al., 1998; Klein &

Purdy, 1993; Yu et al., 2003) as demonstrated with lower energies (6-10 MV) and higher

energies (15-18 MV). Nevertheless, Cobalt-60 is said to create a higher skin dose which

further increases with field size and ranged between 20% - 85% of dmax for the open

beam (Kry et al., 2012). Though the skin-sparing effect relies on many conditions, the

outcome, in general, becomes well pronounced as the photon energy increases. (Martin J.

Butson et al., 1998) in their studies on the central axis of the photon beam relative to dmax

for a field size of 10 x 10 cm2 and obtained measured skin doses greater by 22%, 17%,

and 15.5% corresponding to 6, 10, and 18 MV respectively.

Table 2. 1: Buildup Dose Distribution in Polystyrene for a 10 × 10 cm2 Field

Depth (mm) 60Co 80 cma (cGy) 4 MV 80 cma

(cGy) 10 MV 100 cmb

(cGy) 25 MV 100 cma

(cGy)

0 18.0 14.0 12.0 17.0 1 70.5 57.0 30.0 28.0 2 90.0 74.0 46.0 39.5 3 98.0 84.0 55.0 47.0 4 100.0 90.0 63.0 54.5 5 100.0 94.0 72.0 60.5 6 - 96.5 76.0 66.0 8 - 99.5 84.0 73.0

10 - 100.0 91.0 79.0 15 - - 97.0 88.5 20 - - 98.0 95.0 25 - - 100.0 99.0 30 - - - 100.0

aData from Velkley et al., (1975) bData from F. Khan, Moore, & Levitt, (1973)


15

2.2.3 Effects of Thermoplastic Immobilizers on Surface Dose.

Thermoplastic immobilization devices are mostly used worldwide in head and neck

radiation therapy to substantially improve the precision of treatment delivery. However,

there have been concerns regarding the rise in radiation-induced skin morbidity with the

use of these devices thereby eliminating the skin sparing effect of megavoltage radiation

(Fiorino et al., 1992; Mellenberg, 1995). (Hadley et al., 2004) reported that a rise in

surface dose from 16 to 27% for a 6 MV megavoltage radiation was observed when

thermoplastic masks were used, however, this may increase the skin toxicity and can be

detrimental for the patient’s quality of life. (Mellenberg, 1995) in his studies observed

that, for 6 and 15 MV photon energies, the PDD measured at the surface just behind the

used thermoplastic mask was 35.7% and 24.3% respectively which agrees well with the

PDD measured at 0.6 mm in the beam without the thermoplastic mask (33.8% and

24.5%). In conclusion, he noted that surface dose increases as a gram per square

centimeter relationship for thermoplastic masks. In 2002, research was conducted to

determine the cause of acute skin toxicity in head and neck cancer treatment with

extended-field intensity-modulated radiotherapy by (Lee et al., 2002). They observed that

the estimated surface dose with the thermoplastic mask was constantly larger than those

without the thermoplastic mask. An average rise in dose of approximately 18% was

estimated which was ascribed to the bolus effect of the masks used for IMRT treatment.

Lee et al. later reported that measurements on a phantom presented that the mask had a

“bolus” effect on the surface of the skin and at the same time did not take the skin into

consideration as one of the sensitive structures during treatment planning. They also

added that the use of multiple tangential fields in intensity-modulated radiation therapy


16

plans contributes to an increase in surface dose by approximately 19% and 27% with and

without the thermoplastic mask, respectively. In conclusion, they suggested the skin

should be taken into consideration as one of the sensitive structures during treatment

planning and to reduce the “bolus” effect of the thermoplastic mask was to stretch hard

on the mask to “thin” it out as shown in figures 2.3 – 2.5. (Higgins et al., 2001) in their

studies measured a rise in skin dose as a result of the carbon fiber tables used and it was

as high as 400%. They reported that the presence of the carbon fiber substantially

reduced the skin-sparing effect by a magnitude which was relatively larger for small

beam sizes. They concluded from their research that, the skin dose was approximately

four times as large when the carbon fiber was added to the 10 x 10 beam and

approximately double for the 40 x 40 beam. Works have been done by Hadley et al.

(2004) to investigate the rise in skin dose triggered by the use of thermoplastic mask and

quantify the change between the mask samples presented. The solid water phantom in

conjunction with the Attix parallel plate chamber was used to determine the change in the

dose build-up by measuring the tissue maximum ratios (TMRs). Using dual energies of 6

and 15 MV, measurements were obtained with and without the thermoplastic mask on top

(surface) of the solid water phantom and the effective thickness of equivalent water was

estimated from the TMR curves. They measured the build-up effect to be equivalent to

0.6 to 2.2 mm for the thermoplastic masks that have been extended at varying hole

diameters. The increase in skin dose was projected to rise from 16% and 12% for 6 MV

and 15 MV respectively to 27% to 61% for 6 MV and 18% to 40% for 15 MV with the

thermoplastic mask samples. Below are sample images of thermoplastic masks with


17

figure 2.6 showing the physical effect of the thermoplastic mask on patient’s skin during

a radiation therapy treatment.

Figure 2. 2: An unstretched thermoplastic mask Figure 2. 3: A stretched thermoplastic mask to be used on the solid water phantom

Figure 2. 5: A moulded thermoplastic mask during simulation

Figure 2. 4: A thermoplastic mask fixed to the couch in the treatment room

Figure 2. 6: The effect of thermoplastic mask on skin dose


18

2.2.4 Effect of Field Size on Skin/Surface Dose

(Yadav et al., 2009) studied surface dose estimations for numerous beam modifiers at

different SSD’s for 6 MV photon beam using an acrylic slab phantom and a Markus

0.055 cc parallel plate ion chamber to measure the skin and build-up region doses. They

undertook measurements for motorized wedges, open fields and acrylic block tray fields

for various field sizes of 3 x 3 cm2 to 30 x 30 cm2. They later concluded from their

research that the skin dose for all the beam modifiers increased with increasing field size.

Considering the block trays, they indicated that electrons were eliminated from the

upstream but generated its own new secondary electrons increasing the number of

electrons generated upstream as compared to the ones removed downstream by the tray,

thereby increasing the surface dose. For the 60o motorized wedge fields, they realized

that, at larger field sizes, the increase in skin dose was higher as compared to that for the

open fields. For the physical wedges, electrons were eliminated from upstream and were

produced by itself; with the number generated being less than the ones eliminated for

smaller field size (Kry et al., 2012; Yadav et al., 2009). Increasing field size generally

comes with increasing surface dose which is as a result of the higher electron emissions

from the collimator and air (F. M. Khan, 2014). Figure 2.7 below is a graph showing

relative skin dose against field size for Co-60, 4 MV, and 10 MV photon beams. These

data indicate that the skin-sparing effect is substantially lowered for the higher field sizes.


19

Figure 2. 7: Percentage skin dose against field size. (Data from Velkley, et al., 10 MV data are from Khan FM, et al.)

2.3 Exit Dose

Research has shown that in megavoltage X-rays, the radiation exit dose is influenced by

the dose build-down due to the missing backscatter material beyond the patient’s exit

surface (Kron & Ostwald, 1995). Kron and Ostwald stated that “since the build-down

effect is typically less pronounced as the build-up effect at the entrance portal of the X-

ray beam, the exit dose may contribute significantly to the skin dose and to a possible

unwanted skin reaction”. It is usually complicated to measure the exit dose than the

entrance dose. However, in the absence of the backscatter material, the skin dose is less

than predicted by PDD by approximately 15% (relative difference) for 6 MV or Co-60

beams and is not mostly different for other higher beam energies (Gagnon & Horton,

1979; Klein & Purdy, 1993; Purdy, 1986; Stathakis et al., 2006). Although full scatter

conditions are attained (as predicted by the percentage depth dose) with minimal

material, as well as virtually any immobilizing device behind the patient (Gagnon &

Horton, 1979; Klein & Purdy, 1993; Purdy, 1986).


20

2.4 Radiation Dosimeters for Skin Dose Measurements

A device, tool or a system that is used in measuring or evaluating either indirectly or

directly: absorbed dose, equivalent dose, kerma, exposure or associated quantities of

ionizing radiation is known as a radiation dosimeter. It can provide a measurement that is

a degree of the average absorbed dose deposited in its sensitive volume by the radiation.

It is referred to as a dosimetry system in conjunction with its reader. For a device to work

as a dosimeter, it must have at least one physical characteristic that is dependent on the

measured quantity and that can be used for measuring the radiation with correct

calibration. Nevertheless, to be suitable, the dosimeters must show various required

features. For instance, the dimensions of the dosimeter should be as small as possible

along the incident beam, due to the steep dose gradient in the build-up region. Hence, an

extrapolation chambers is the suitable instruments for such the measurements as indicated

by (Khan, 2014). However, only a few centers have these radiation dosimeters available

and as an alternative, the fixed plane-parallel ion chambers are usually used for this

purpose. In addition to ionization chambers, thin layers (< 0.5 mm) of TLD material

(Rapley, 2006) and radiochromic films such as the GafChromic films are available for

measurements of skin dose. The film is first calibrated within the dose range required

before use (Butson et al., 1999; Khan, 2014). In this study, the choice of the ionization

chamber and GafChromic films were made due to its availability at the facility.

2.4.1 Ionization Chambers

Ionization chambers are types of radiation dosimeters which are employed in both

radiation therapy and diagnostic radiology for the measurement of doses. They come in


21

numerous sizes and shapes subject to a particular requirement, but usually they possess

these features: It is fundamentally a gas-filled cavity that is enclosed by a conducting

outer layer with a collecting electrode at the center which is separated with a high-quality

insulator in order to decrease the leakage current when a polarizing voltage is applied to

the chamber. Additional chamber leakage is reduced by means of a guard electrode that

cuts off the leakage current and permits it to move to the ground, avoiding the collecting

electrode. Better charge collection due to enhanced field consistency in the

sensitive/active volume of the chamber is another advantage. However, (Podgorsak &

Rosenberg, 2005) stated that temperature and pressure corrections are required during

measurements with open-air ion chambers to help explain the variation in air density in

the chamber volume as a result of a change in ambient temperature and pressure.

Particularly, for skin dose measurements are the fixed plate ionization chambers and the

extrapolation chambers (Andreo et al., 1995; Khan, 2014; Rawlinson et al., 1992). In this

work, a Roos Chamber type 34001, used for dose or dose rate measurement for the

measuring quantity “absorbed dose to water” in solid water phantoms and used for the

determination of relative depth dose curves of megavoltage beams was used in

conjunction with an electrometer. The Roos chamber is said to be waterproof, fully

guarded having a 15 mm width collector and a nominal sensitive volume of 0.35 cm3.

2.4.2 Electrometers

Electrometers are devices used for measuring small currents (approximately ≤ 10-9 A).

When used in combination with an ion chamber, it gives a high gain, negative feedback,


22

operational amplifier with a standard resistor or a standard capacitor in the feedback path

to determine the ion chamber current or charges collected over a time range.

2.4.3 Radiochromic Film

Several methods and procedures have been developed for the measurement of a patient’s

skin dose using different dosimeters. This research seeks to use a new type of film in

radiotherapy dosimetry known as the radiochromic film. GafChromic films are said to be

the most commonly used films in dosimetry (e.g. HD-810 film, DM-1260, EBT, EBT2,

EBT3, MD-55 and MD-55-2 film with an effective depth of skin dose measurement at

0.17mm ± 0.03mm). They are said to be colourless films with closely tissue equivalent

composition (11.2% nitrogen, 19.2% oxygen, 60.6% carbon, and 9.0% hydrogen) which

changes colour to dark blue upon radiation exposure. The unexposed radiochromic film is

said to be colourless and changes colour to shades of blue due to its composition of a

special dye that polymerizes as a result of its interaction with radiation. After the

absorption of light by the polymer, the light transmission through the radiochromic film

can be measured using an appropriate device known as a densitometer. Radiochromic

film requires no chemical, physical or thermal processing to bring out or stabilize this

colour. Studies have shown that the film’s ability to respond to radiation exposures less

than 50 mGy is not noticeable making the flatbed scanner incapable of picking up the

small changes in film color (Lewis, 2009). Radiochromic films used in dosimetry is said

to have some potentials over radiographic films such as ease of use; eliminates the need

for darkrooms, film cassettes or film processing (chemicals); insensitive to ambient

conditions and improved energy characteristics making it useful at higher doses


23

(Podgorsak & Rosenberg, 2005). Radiochromic films are relative dosimeters and must be

well calibrated before used for dosimetry, however, with proper calibration and taking

into account the environmental conditions, a precision better than 3% can be archived

(Khan, 2014). The film is also used for field size alignment and shaping of radiation

fields, detecting radiation leakage around the collimator head and positioning of special

radiation fields (Niroomand-Rad et al., 1998). With careful handling and calibration of

the film in accordance with the overall recommendations outlined by the manufacturer’s

specifications as stated in the AAPM TG-55 report by (Niroomand-Rad et al., 1998) it

should be possible to achieve a precision of better than 3% for dosimetric work.

2.4.3.1 Introduction

A GafChromic [International Specialty Products (ISP), Wayne, NJ] EBT3 film which is

specifically developed to address the need of a medical physicist working in the

radiotherapy setting and a type of a radiochromic film was used for this study. The active

range of the film is constructed for greatest operation in the dose range from 0.2 to 10

Gy. This makes it good for various applications in intensity modulated radiotherapy

(IMRT), VMAT and brachytherapy. Like the previous versions, EBT3 is said to be self-

developing, requiring neither developer nor fixer. Some of the main technical features of

the GafChromic EBT3 film include:

It is a near tissue equivalent material which requires no post-exposure treatments since it

develops in real time. It has a dynamic dose range of 0.1 to 20 Gy with 0.2 to 10 Gy

being its optimum dose range although it has a slight response variation from 100 keV

into the MV range. The film has a high spatial resolution, hence, can resolve features


24

down to 25 µm or less. It is water-resistant and usable with water phantoms with a new

technology of incorporating a marker dye in its active layer to decrease UV/visible light

sensitivity and also enable using multi-channel dosimetry for non-uniformity correction.

The film is said to be stable at temperatures up to 600C.

2.4.3.2 Configuration and Structure of GafChromic EBT3

The structure of the GafChromic EBT3 film is shown in figure 2.8 below. The EBT3 film

is made up of an active layer, nominally 28 μm thick, sandwiched between two 125 μm

matte-polyester substrates. This active layer comprises a marker dye, the active

component, stabilizers and other components giving the film its close energy independent

response. The thickness of the active layer will differ to some extent between different

production lots.

Figure 2. 8: Structure of GafChromic EBT3 Dosimetry Film (Actual thickness may vary slightly).

(Sipilä et al., 2016) in their studies indicated that “the surface of the radiochromic film is

covered with tiny silicone spheres eliminating Newton’s rings artifact in the image”.

Further explanation of the film feature is described by (David Lewis et al., 2012) in their

research.


25

2.4.3.3 Medical Applications

Radiochromic films are said to be comparatively unresponsive to ionizing radiation

compared to frequently used detectors in medical applications making it ideal for

dosimetry where high radiation doses are applied (Niroomand-Rad et al., 1998). The

application of radiochromic film in the medical fields is vastly extending from high-dose

gamma ray exposures in brachytherapy to low-dose clinical valuation in-vivo such as

conventional radiation therapy of breast cancer patients. Below are three medical

applications of a GafChromic (radiochromic) films (Martin J. B. et al., 1999; Stevens et

al., 1996).

i. Proton Dosimetry

(Nichiporov et al., 1995; Vatnitsky, 1997 and Vatnitsky et al., 1997) indicated that one of

the useful detectors in clinical proton beams for dosimetry measurements is the

radiochromic films. It possesses a fundamental composition equivalent to that of water,

which decreases its sensitivity to photon energy for an application dealing with the

determination of dose delivered to water as is given in TG-55 (Niroomand-Rad et al.,

1998).

ii. Skin and Surface Dosimetry

Due to the low energy reliance of the film, relatively small effective thickness and the

ability to produce a two-dimensional dose map which other skin dosimeters in

radiotherapy is currently lacking, the radiochromic film is a detector for choice. These

applications include in-vivo dosimetry as well as phantom studies for dose calculation at


26

the skin surface, dermal and basal cell layers as well as the subcutaneous layers as

indicated by (Niroomand-Rad et al., 1998).

iii. Brachytherapy

In brachytherapy, one of the advantages is the sharp or steep dose fall-off outside the

tumor or target volume. From the inverse square law, radiation dose at a point close to the

radiation source can be very high, which becomes a problem with the use of conventional

detectors. The low sensitivity and high resolution of the radiochromic film give it an

advantage over the other detectors which makes it useful for dosimetry near these high

activity sources (Niroomand-Rad et al., 1998).

2.4.3.4 Advantages of GafChromic Films

Below are some of the advantages of using GafChromic films for dosimetry.

The GafChromic film gives permanent absolute values of absorbed dose with a

satisfactory accuracy and precision. It provides a larger area for dosimetry; including

beam profiles. It has a higher spatial resolution and easy to handle and analyze. Non-

uniformity correction is possible when using multi-channel dosimetry. The film is water-

resistant and usable with water phantoms. It needs no darkrooms, film processing

(chemicals) or cassettes due to its real-time developments without post-exposure

treatment. The film is said to be insensitive to visible light.


27

CHAPTER THREE

MATERIALS AND METHODS

The methodologies adopted to assess the effects of thermoplastic immobilization mask on

patient’s skin and the build-up region doses in the head and neck region for high energy

photon beams using a Roos Chamber type 34001 coupled with PTW Freiburg UNIDOS

Electrometer and EBT3 GafChromic films for different field sizes are presented in this

chapter. Known procedures for the calibration of GafChromic EBT3 films for assessing

skin dose with solid water phantom together with the dose measurements are outlined in

this chapter.

3.1 Materials

The materials used in this study include; a medical linear accelerator unit (figure 3.1),

anthropomorphic phantom, plastic water (PMMA) phantom, PTW Freiburg UNIDOS

electrometer, Roos chamber type 34001, thermoplastic masks, EBT3 GafChromic film,

EPSON STYLUS CX5900 flatbed scanner, water bath, diode, diode reader, digital

thermometer, and barometer. Some specific materials are presented with details below.

3.1.1 Medical Linear Accelerator

This is a device most commonly used for the treatment of cancer with external beam

radiation. The linear accelerator can be used to treat all body parts or organs of a patient

undergoing radiation therapy treatment. The device uses high radio-frequency (RF)

electromagnetic waves to accelerate charged particles (electrons) to interact with a heavy

metal target and because of these collisions, high energy X-rays (photons) are emitted


28

from the target in a linear path, within a tube-like structure called the accelerator

waveguide. The LINAC delivers these high energy X-rays or electrons to the target tumor

as they exit the tube. These treatments can be planned in such a way that they destroy the

cancerous cells while sparing the neighboring healthy tissues, thus achieving the desired

therapeutic ratio. At Sweden Ghana Medical Centre, there is one Elekta Synergy platform

linear accelerator unit in the radiotherapy department. The LINAC delivers both electron

and photon energies. It produces two different photon energies (that is, 6 MV and 15

MV) and three different electron energies (that is, 6 MeV, 10 MeV, and 15 MeV)

respectively. A specialized multi-leaf collimator of 2 x 40 leaf’s and 1 cm leaf width is

incorporated in the treatment head at the isocenter. The LINAC is used in treating all

body sites, using Intensity-Modulated Radiation Therapy (IMRT), Conventional

Techniques, Volumetric Modulated Arc Therapy (VMAT), and Stereotactic Radiosurgery

(SRS). The treatment couch used at the Sweden Ghana Medical Center is the precise and

hexapod type. Figure 3.1 shows a pictorial view of the Synergy Platform Linear

Accelerator unit at SGMC.


29

Figure 3. 1: Medical Linear Accelerator at Sweden Ghana Medical Centre (SGMC)

3.1.2 Ionization Chamber

The type of ion chamber used for this study was the Roos type chamber 34001 which is a

vented plane-parallel ionization chamber for connection to precision electrometers

according to IEC 60731 for measurements in electron beams. It is used as reference

chamber in absolute dosimetry, that is, in radiation therapy, the Roos chamber is used for

dose or dose rate measurement for the measuring quantity 'absorbed dose to water' in

solid state phantoms or water phantoms. The detector has a wide guard ring, which

excludes any perturbation effect even at low electron energies and in the build-up range

of photon beams. The electron energy response of the detector in the range of 2 MeV to

45 MeV is provided by the stopping power correction. The Roos type chamber requires

no corrections due to its design. The Roos chamber as shown in figure 3.2 can also be

used for measuring the relative depth dose curves of high energy photon beams. The


30

Roos chamber is waterproof, fully guarded, has a 15 mm diameter collector, with a

supposed sensitive volume of 0.35 cm3.

Figure 3. 2: Roos Chamber type 34001

Using the Roos Ionization Chamber

The ion chamber was connected to the electrometer and switched on. The high voltage

was correctly adjusted on the electrometer and allowed for about 15 minutes to stabilize

before the measurements were started. The correction factor for air density was noted and

used to correct for the measured values. Lastly, a zero adjustment was performed

followed by the measurements.

Application in the Solid Water Phantom

Special chamber plates are available for measurements with the detector in a slab

phantom. The chamber plates are available in the form of twin plates. The active point of

measurement (reference point) of the detector in this plate is approximately 1 mm below

the surface of the twin plate.


31

Inserting the Detector into the twin plate

The detector is inserted into the twin plate so that the ‘crosshairs’ is visible and points

towards the radiation source as in figure 3.3. Figure 3.4 also shows the final stage of

inserting the detector into the twin plate which is placed on the treatment couch for

measurements.

Figure 3. 3: Inserting/removing the Roos Chamber

Figure 3. 4: Twin plate with inserted Roos chamber

Removing the Detector from the Twin Plate

To remove the Roos Chamber after measurements, the plate is lifted with the ‘crosshairs’

and the detector is carefully removed (refer to figure 3.3).

3.1.3 Electrometer

The electrometer used was the UNIDOS (PTW, Freiburg, Germany) model with serial

number 000820. Figure 3.5 provides an image of the front view of the electrometer. The


32

electrometer was calibrated together with the ion chamber mentioned above. It was used

to quantify the charges that have been collected by the ionization chamber in nC/minute

and the measured values were then corrected for temperature and pressure variations

respectively.

Figure 3. 5: PTW UNIDOS Electrometer

3.1.4 Diodes

Diodes are inexpensive, rugged and easy to use as they require no bias. Although they are

smaller in size as compared to other ion chambers, diodes are preferred for clinical

dosimetry due to its higher relative sensitivity and quick response (1 – 10 µs). However,

diodes are said to suffer radiation damage, so they require occasional recalibration. The

diodes in this work were used to measure doses at the skin surface and dmax in “real time”

and recorded accordingly (figure 3.6).

Figure 3. 6: Diode


33

3.1.5 Water Bath

To make a thermoplastic mask pliable or moldable to obtain the contours of the patient’s

anatomy, the mask is first placed in a water bath above a specific temperature to make it

soft and solidifies upon cooling after molding. The water bath shown in figure 3.7 can

modulate/regulate the temperature of the water.

Figure 3. 7: Water Bath

3.1.5 Thermoplastic Immobilization Mask

A thermoplastic mask is a plastic material made up of two parts; a sheet of thermoplastic

material and a rigid frame at the edge. When immersed in warm water above a

temperature, the mask becomes soft and pliable, after which it is placed over the patient

and moulded to the patient surface. As the mask cools down, a rigid replication of the

patient’s anatomy is formed. The masks were attached to a rigid frame which mounts to

the treatment couch. Thermoplastic masks are available commercially to immobilize

different patient’s treatment sites. In this research, the surface dose was measured with

and without the head and neck mask. The masks used in this study were placed on the

phantom setup to cover the surfaces.


34

3.1.6 GafChromic Films

The radiochromic film used here was a GafChromic EBT3 film (lot # 04201601), with

known dimensions of 8” x 10”. In view of the procedures stated in the AAPM TG-55

report by (Niroomand-Rad et al., 1998), the film was handled and used in accordance

with the overall recommendations outlined by the manufacturer’s specifications in order

to preserve its integrity. (Podgorsak & Rosenberg, 2005) described GafChromic film as a

colourless film with nearly tissue equivalent composition that develops a dark blue colour

upon its exposure to radiation and it is intended for the determination of absorbed doses

of ionizing radiation with a dynamic range intended for greatest operation in the dose

range from 0.2 Gy to 10 Gy, making it appropriate for different applications in IMRT,

VMAT, and Brachytherapy. It has the ability of self-developing right after exposure and

requires neither a developer nor a fixer. During the estimation of doses considerably

larger than 10 Gy, EBTXD or MDV3 are chosen while the measurement of still higher

dose uses the HDV2.

3.1.7 ImageJ Software

The ImageJ software is a freely available Java-based image processing and analysis

software developed/established at the National Institutes of Health (NIH). This software

was built with an open architecture that offers extensibility via Java plugins and

recordable macros (Girish, V., Vijayalakshmi, 2004) by using objective physical

characteristics of the imaging system such as spatial resolution, contrast, and noise. The

user-written plugins make it possible to solve almost any image processing and analysis


35

problems (Eliceiri & Rueden, 2005). Figure 3.8 shows the interface of the ImageJ

software.

Figure 3. 8: Image J software interface

3.1.8 Other Materials

Figure 3.9, 3.10, 3.11, and 3.12 are images of a barometer, diode reader, solid water

phantom with known dimensions of 30 x 30 cm2, and a thermometer respectively, which

were used for the setup.

Figure 3. 9: Barometer Figure 3. 10: Diode reader

Figure 3. 11: Solid water phantom Figure 3. 12: Thermometer


36

3.2 Experimental Method

In radiotherapy, before any measurement is clinically done, it is required that thorough

quality control tests are performed. These tests were performed on all equipment before

any measurement was taken, and all the equipment were allowed to acclimatize to the

room’s conditions. The chamber was pre-irradiated for about 5 minutes to remove all

traces of stray charges and checked for stability. The solid water phantom was setup on

the LINAC’s couch, perpendicular to the beams central axis using the isocentric

technique. The Roos chamber was positioned, coupled with the UNIDOS electrometer at

the required depth within the phantom and used to collect and measure the charges

respectively. The initial temperature and pressure were recorded using the thermometer

and the barometer respectively.

3.2.1 Part 1:

3.2.1.1 Measurement Without Thermoplastic Mask

Using a varying field size of 5 x 5, 10 x 10, 15 x 15, and 20 x 20 cm2 without a

thermoplastic mask as illustrated in figure 3.13, the ionization chamber was irradiated

with 100 MU (1 Gy) for the open beam. The charges collected by the ion chamber was

measured and recorded using the electrometer. Three successive measurements were

taken for each field size and their average values estimated and recorded. The final

temperature and pressure were recorded using the thermometer and the barometer

respectively. The experiment was repeated for other depths of 0, 1.6 (dmax), 4, 6, 8, and 10

cm for the open beam. Temperature and pressure correction was performed for all charge

measurements.


37

Figure 3. 13: Setup without a thermoplastic mask

3.2.1.2 Measurement on an Unstretched Thermoplastic Mask

Using a varying field size of 5 x 5, 10 x 10, 15 x 15, and 20 x 20 cm2 with an unstretched

mask as illustrated in figure 3.14, the ionization chamber was irradiated with 100 MU (1

Gy) for the open beam. The charges collected by the ion chamber was measured and

recorded using the electrometer in charge mode. Three successive measurements were

taken for each field size and their average values estimated and recorded. The final

temperature and pressure were recorded using the thermometer and the barometer

respectively. The experiment was repeated for other depths of 0, 1.6 (dmax), 4, 6, 8, and 10

cm for the open beam. Temperature and pressure correction was performed for all charge

measurements.

Figure 3. 14: Setup with an unstretched thermoplastic mask


38

3.2.1.3 Measurement on a 5 cm Stretched Thermoplastic Mask

The thermoplastic mask was placed into a water bath and heated to a temperature of

about 70 0C for about 1 minute. The mask was removed from the water bath, stretched

slightly to a length of 5 cm and fixed on the surface of the phantom as shown in figure

3.15 below and allowed to harden for about 5 minutes. Using a varying field size of 5 x 5,

10 x 10, 15 x 15, and 20 x 20 cm2 with the 5 cm stretched mask, the ion chamber was

irradiated with 100 MU (1 Gy) for the open beam. The charges collected by the ion

chamber was measured and recorded using the electrometer in charge mode. Three

successive measurements were taken for each field size and their average values

estimated and recorded. The final temperature and pressure were recorded using the

thermometer and the barometer respectively. The experiment was repeated for other

depths of 0, 1.6 (dmax), 4, 6, 8, and 10 cm for the open beam. Temperature and pressure

correction was performed for all charge measurements.

Figure 3. 15: Setup with a 5 cm stretched thermoplastic mask


39

3.2.1.4 Measurement on a 10 cm Stretched Thermoplastic Mask

The thermoplastic mask was re-immersed into the water bath, removed, stretched fully to

a length of 10 cm to obtain a very small thickness. Using a varying field size of 5 x 5, 10

x 10, 15 x 15, and 20 x 20 cm2 with the 10 cm stretched mask, the ion chamber was

irradiated with 100 MU (1 Gy) for the open beam. The charges collected by the ion

chamber was measured and recorded using the electrometer in charge mode. Three

successive measurements were taken for each field size and their average values

estimated and recorded. The final temperature and pressure were recorded using the

thermometer and the barometer respectively. The experiment was repeated for other

depths of 0, 1.6 (dmax), 4, 6, 8, and 10 cm for the open beam. Temperature and pressure

correction was performed for all charge measurements.

3.2.1.5 Temperature and Pressure Correction

Since calibration factors are for standard environmental conditions of temperature at T0 =

22 0C and P0 = 110.33 kPa, charge readings were corrected to standard environmental

conditions using the equation:

PPT = ……………………. (3.1)

Where T, P, and PPT is the temperature in degrees Celsius, the pressure in kilopascals and

the correction factor respectively. The average readings were then multiplied by the

correction factor, PTP to acquire the corrected reading:

Mcor = M x PTP ……………………. (3.2)

Where M is the average ionization chamber reading and Mcor is the corrected average

ionization chamber reading.


40

3.2.1.6 Measurement of The Thermoplastic Mask Factor, TF.

To know and correct for the effects the thermoplastic mask has on the skin dose, the

thermoplastic factor variation with depth and field size were determined. Just like the tray

factor, the thermoplastic mask factor is expressed as a ratio of the corrected ion chamber

readings with mask I1 to that without mask I0. That is,

TF = I1/I0 ………………… (3.3)

3.2.2 Part 2:

3.2.2.1 GafChromic Film Calibration

A solid water phantom (PTW, Freiburg, Germany) of dimensions 30 x 30 cm2 was used

for the calibration with 10 cm of the build-up material above and below the GafChromic

film. The field size of 10 x 10 cm2 was used at the isocenter and SSD was set at 100 cm.

The GafChromic film samples were cut (2 x 3 cm2) and irradiated perpendicularly to the

beams central axis for the 6 MV photon beam. A calibrated ion chamber was introduced

in the water phantom below the film level to determine the LINAC’s output during the

irradiation process and measured the absorbed dose deposited to the film by employing

the IAEA-TRS 398 protocol (Musolino, 2000). To acquire a calibration curve, the films

were irradiated with the LINAC (using 6 MV) of uniform radiation at dose levels of 0,

0.20 Gy, 0.40 Gy, 0.80 Gy, 1.60 Gy, 2.40 Gy, 3.20 Gy, 4.00 Gy, and 5.00 Gy. The cut

films were labeled with respect to the above radiation doses.


41

3.2.2.2 Measurement of Skin Dose Using GafChromic Films

Skin dose measurements were carried out using GafChromic films. Firstly, the

GafChromic film was cut into small pieces with dimensions of 2 x 3 cm2 each and

labeled as A, B, and C respectively. Film A was positioned on top of the phantom, film B

at the dmax of 1.6 cm for the 6 MV photon beam, and film C placed at a depth of 10 cm as

shown in figure 3.16 below. GafChromic film C was used to determine the percentage

depth dose (PDD) at a reference depth of 10 cm within the phantom and compared with

the PDD at a depth of 10 cm for the LINAC beam. Using the isocentric technique, the

labeled films, A, B, and C were placed at the surface, dmax and at a reference depth of 10

cm respectively. Using a varying field size of 5 x 5, 10 x 10, 15 x 15, and 20 x 20 cm2,

the films were then irradiated/exposed with 100 MU (1 Gy) for the open beam. For each

field sizes, the set of films A, B, and C were kept together in an envelope labeled with the

various field sizes.

Figure 3. 16: Positioning of the film set A, B, and C at the surface of the phantom, at the dmax of 1.6 cm for

the 6 MV photon beam, and at a depth of 10 cm respectively


42

3.2.2.3 Measurement of Skin Dose with The Introduction of Thermoplastic

Immobilization Devices

The thermoplastic mask was positioned in the path of the photon beam on the

GafChromic film placed on top of the solid water phantom. The procedure was repeated

as in the setup with and without the immobilizing device for the different depths and field

sizes for the open beam. The set of films A, B, and C were also kept in an envelope

labeled with the field size, and the presence of the thermoplastic mask (whether stretched

or unstretched). The irradiated films with one unexposed film serving as control were

stored for 24 hours for post-radiation exposure development. Lastly, these films were

stored at a temperature of 22 0C ± 2 0C to enable stable temperature range during

irradiation, storage, and scanning.

3.2.2.4 Scanning of The Irradiated Films

The exposed films were stored in envelopes for at least 24 hours at a particular

temperature range to safeguard the darkening effect stabilization. Before the scanning

process, the large scanner glass of the flatbed was carefully cleaned with a lint-free cloth

moistened with isopropyl alcohol and a minimum of three empty scans were made to

warm up the lamp of the scanner. This is to take into account the drift of the EPSON

scanner on short-term (Menegotti, Delana, & Martignano, 2008). The films were

removed and placed in the scanner as shown in fig 3.17 and the images were acquired in

the landscape mode, as suggested by the manufacturer, because (Borca et al., 2013)

explained that the lateral response artifact on EPSON STYLUS CX5900 scanners is

smaller in this orientation as compared to the portrait orientation, utilizing its associated


43

software package “EPSON scan,” used in professional mode with the image adjustments

and color correction turned off, with a spatial resolution of 72 dpi in the 48 bit RGB

mode (that is, 16 bit per channel) (Menegotti et al., 2008). The acquired data were saved

in uncompressed tagged image file format (TIFF).

3.2.2.5 Reading of The Irradiated Films (Using ImageJ)

The raw images of the irradiated films were imported into ImageJ analysis software for

additional image processing. Film responses were then converted into doses (irradiated

and unirradiated) and recorded in an excel worksheet. Figure 3.18 shows the ImageJ

software interface with a scanned film image split into the RGB channels during the

image processing and analysis.

Scan direction Central placement

in this direction

Figure 3. 17: Film placement on the scanner


44

Figure 3. 18: ImageJ software interface with a scanned film image split into the RGB channels

The net optical densities of the data obtained from the mean RGB values were calculated

using the following equation:

3.2.3 Part 3:

3.2.3.4 Dose Verification Using the Anthropomorphic Phantom

Here the anthropomorphic phantom and the diode were used for the dose verification of

the treatment plan. The phantom was first scanned using the Siemens SOMATOM

Emotion 16 CT-scanner at the head and neck region in the head first supine (HFS)

position. The acquired CT images were then imported into the TPS and a plan for the


45

target region was planned to use the 6 MV photon energy. A prescribed dose of 2 Gy was

planned at the isocenter to the skin surface and the dmax at an SSD of 90 cm using 10 x 10

and 20 x 20 cm2 field sizes. In the treatment room, the phantom was then irradiated as

planned and the diode measured and displayed the output on the diode monitor as shown

in figure 3.19. A comparison of the measured dose values and the planned dose values

were done to assess the difference between the planned and delivered dose.

The Siemens SOMATOM Emotion 16 CT- scanner used for scanning the phantom

Setting up the phantom with the mask for irradiation

CT images imported onto the TPS for planning.

Scanning Planning

Treatment

Figure 3. 19: Treatment planning process for the dose verification


46

CHAPTER FOUR

RESULTS AND DISCUSSION

4.1 Introduction

The use of the thermoplastics masks for immobilization during treatment of patients can

be analyzed on the basis of percentage depth dose (PDD) effect, and surface dose effect

on the patient. These skin dose measurements were made using the Roos type ionization

chamber coupled with the UNIDOS electrometer and GafChromic EBT3 films to

compare them with the values obtained from the ionization chamber. This chapter moves

on to the presentation of the findings produced by the original quantitative analysis

conducted as a part of this research. The raw data from all the measurements are

presented in the appendices.

4.2 Dose Determination from Ion chamber

The absorbed dose at varying depths (that is, 0, 1.6, 4, 6, 8, and 10 cm) and field sizes

(that is, 5 x 5, 10 x 10, 15 x 15, and 20 x 20 cm2) of the solid water phantom was

determined and tabulated. Tables 4.1 – 4.4 present the measured absorbed dose values

using the Roos type ionization chamber coupled with the UNIDOS electrometer for

different field sizes at different depths.


47

Table 4. 1: Measured Absorbed dose (Gy) values for various field sizes at different depths without the mask.

It can be observed from Table 4.1 that in the absence of the thermoplastic mask for a field

size of 5 x 5 cm2, the skin dose elevated from 0.39 to 0.97 Gy at dmax (1.6 cm) and later

dropped to 0.71 Gy at the depth of 10 cm. Taking into consideration the skin dose, dose

at maximum depth, and dose at a reference depth of 10 cm, it can be seen that with

increasing field size from 5 x 5 to 20 x 20 cm2, the skin dose increased from 0.39 to 0.57

Gy; dmax also increased from 0.97 to 1.07 Gy; and at the depth of 10 cm the dose

increased from 0.71 to 0.87 Gy respectively. For the various field sizes, the skin dose

increased by a factor of 0.06%. These results conform with literature (Hadley, Kelly, &

Lam, 2005; F. M. Khan, 2014; Kry et al., 2012) since the dose gradually rises to a

maximum depth where the majority of the radiation dose is deposited and falls with

increasing depth within the phantom.

Table 4. 2: Measured Absorbed dose (Gy) values for various field sizes at different depths with the unstretched mask.

Depth (cm) Field Size (cm2) (Gy) 5x5 10x10 15x15 20x20

0 0.60 0.66 0.72 0.77 1.6 0.97 1.02 1.05 1.07 4 0.90 0.96 0.99 1.02 6 0.83 0.90 0.94 0.97 8 0.76 0.84 0.88 0.92 10 0.70 0.78 0.83 0.86


0 0.39 0.45 0.51 0.57 1.6 0.97 1.02 1.05 1.07 4 0.90 0.96 1.00 1.02 6 0.84 0.91 0.94 0.97 8 0.77 0.85 0.89 0.92 10 0.71 0.79 0.83 0.87


48

It can be observed from Table 4.2 that with the presence of the unstretched thermoplastic

mask for a field size of 5 x 5 cm2, the skin dose increased from 0.39 Gy (without mask)

to 0.60 Gy by a factor of 0.2%. This rise in skin dose was ascribed to the “bolus” effect of

the mask which shifted the dmax closer to the skin surface. Nevertheless, the dose at

maximum depth and at the reference depth of 10 cm did not change significantly. Taking

into consideration the skin dose, dose at maximum depth, and dose at a depth of 10 cm, it

can be stated that with increasing field size from 5 x 5 to 20 x 20 cm2, the skin dose

increased from 0.60 to 0.77 Gy; dmax also increased from 0.97 to 1.07 Gy; and at the

depth of 10 cm the dose increased from 0.70 to 0.86 Gy respectively. For the various

field sizes, the skin dose increased by a factor of 0.06%. These results as well conform

with literature (Hadley et al., 2005; F. M. Khan, 2014; Kry et al., 2012) since the dose

gradually rises to a maximum depth where the majority of the radiation dose is deposited

and falls with increasing depth within the phantom.

Table 4. 3: Measured Absorbed dose (Gy) values for various field sizes at different depths with the 5 cm stretched mask.


0 0.52 0.58 0.64 0.70 1.6 0.99 1.04 1.07 1.09 4 0.93 0.99 1.02 1.04 6 0.86 0.93 0.97 0.99 8 0.79 0.87 0.91 0.94 10 0.73 0.81 0.86 0.89

It can as well be observed from Table 4.3 that with the presence of the 5 cm stretched

thermoplastic mask for a field size of 5 x 5 cm2, the skin dose decreased from 0.60 Gy

(with the unstretched mask) to 0.52 Gy by a factor of 0.08%. This decrease in skin dose


49

was ascribed to the reduction in the thickness of the thermoplastic mask as a result of the

stretching. However, comparing the dose at maximum depth and at the reference depth of

10 cm of the unstretched mask with the 5 cm stretched mask, it can be said that the doses

changed significantly by factors of 0.02% and 0.03% respectively. Taking into

consideration the skin dose, dose at maximum depth, and dose at a reference depth of 10

cm, it was observed that with increasing field size from 5 x 5 to 20 x 20 cm2, the skin

dose increased from 0.52 to 0.70 Gy; dmax also increased from 0.99 to 1.09 Gy; and at the

depth of 10 cm the dose increased from 0.73 to 0.89 Gy respectively. These results as

well conform with literature (Hadley et al., 2005; F. M. Khan, 2014; Kry et al., 2012)

since the dose gradually rises to a maximum depth where the majority of the radiation

dose is deposited and falls with increasing depth within the phantom.

Table 4. 4: Measured Absorbed dose (Gy) values for various field sizes at different depths with the 10 cm stretched mask

Depth (cm) Field Size (cm2) (Gy)

5x5 10x10 15x15 20x20 0 0.44 0.51 0.57 0.63

1.6 0.99 1.04 1.07 1.10 4 0.93 0.99 1.02 1.04 6 0.86 0.93 0.97 1.00 8 0.80 0.87 0.91 0.94 10 0.73 0.81 0.86 0.89

It can as well be observed from Table 4.4 that with the presence of the 10 cm stretched

thermoplastic mask for a field size of 5 x 5 cm2, the skin dose decreased from 0.52 Gy

(with the 5 cm stretched mask) to 0.44 Gy by a factor of 0.07%. This decrease in skin

dose was as well ascribed to the further reduction in the thickness of the thermoplastic

mask as a result of the stretching. However, comparing the dose at maximum depth and


50

at a depth of 10 cm of the 5 cm stretched mask with the 10 cm stretched mask, it can be

said that the doses did not change significantly. Taking into consideration the skin dose,

dose at maximum depth, and dose at a reference depth of 10 cm, it was observed that

with increasing field size from 5 x 5 to 20 x 20 cm2, the skin dose increased from 0.44 to

0.63 Gy; dmax also increased from 0.99 to 1.09 Gy; and at the depth of 10 cm the dose

increased from 0.73 to 0.89 Gy respectively. These results also conform with literature

(Hadley et al., 2005; F. M. Khan, 2014; Kry et al., 2012) since the dose gradually rises to

a maximum depth where the majority of the radiation dose is deposited and falls with

increasing depth within the phantom.

4.2.1 Graphical Representations of Absorbed Dose against Depth

The figures 4.1 – 4.4 are the graphical representations of the absorbed doses obtained

using the ionization chamber against depth as tabulated in Table 4.1 – 4.4 respectively.

Figure 4. 1: A graphical representation of Absorbed dose against Depth for no thermoplastic mask.


51

Figure 4. 2: A graphical representation of Absorbed dose against Depth for the unstretched thermoplastic

mask.

Figure 4. 3: A graphical representation of Absorbed dose against Depth for the 5 cm stretched

thermoplastic mask.


52

Figure 4. 4: A graphical representation of Absorbed dose against Depth for the 10 cm stretched

thermoplastic mask.

4.3 Percentage Depth Dose (PDD) Determination

The determination of the percentage depth dose with and without the thermoplastic mask

made it easy to assess the build-up effect of the thermoplastic mask on the skin. Figures

4.5 – 4.7 show a graph of percentage depth dose against depth with or without the mask

for a 10 x 10 cm2 field size.


53

Figure 4. 5: A graph of PDD against Depth for a 10x10 field size with an unstretched mask and without a

mask.

Figure 4. 6: A graph of PDD against Depth for a 10x10 field size with a 5 cm stretched mask and without a

mask.


54

Figure 4. 7: A graph of PDD against Depth for a 10 x 10 field size with a 10 cm stretched mask and without

a mask.

Due to the presence of the thermoplastic mask during measurements, the depth dose in

the build-up region increased, subsequently shifting of the dmax to increasingly lower

depths to the skin surface. For a field size of 10 x 10 cm, an increase in skin dose of 0.21,

0.13 and 0.05 Gy were estimated for the unstretched mask, the 5 cm stretched mask and

the 10 cm stretched mask respectively, which was ascribed to the “bolus” effect of the

thermoplastic masks.

4.3.1 Variation of Percentage Depth Dose (PDD) with Field Size and Depth.

Field size, which is a scatter factor is proportional to percentage depth dose. The

influence of the scattered radiation to the absorbed dose at a point in the patient or

phantom increases with respect to field size and therefore the increase in scattered dose


55

occurs at greater depths than at the depth of maximum dose (dmax) causing an increase in

dose. The PDD increased from the surface to a dmax (1.6 cm) and then decreased with

increasing depth. The variation in percentage depth dose as a function of the field size

and depth for the various thicknesses of the thermoplastic mask are represented in figures

4.8 - 4.11.

Figure 4. 8: A graph showing the variation of PDD versus depth and field size for measurements without

the thermoplastic mask.


56

Figure 4. 9: A graph showing the variation of PDD versus depth and field size for measurements with an

unstretched thermoplastic mask.

Figure 4. 10: A graph showing the variation of PDD versus depth and field size for measurements with a 5

cm stretched thermoplastic mask.


57

Figure 4. 11: A graph showing the variation of PDD versus depth and field size for measurements with a 10

cm stretched thermoplastic mask.

4.4 Determination of Skin Dose from PDD values

From figure 4.12, the percentage depth dose measurements without the thermoplastic

mask at 0.0 cm depth was compared to the percentage depth dose measurements at a

depth of 0.0 cm with the various thermoplastic masks (that is, unstretched, 5 cm and 10

cm stretched masks). It can be seen that the presence of the thermoplastic mask as an

immobilizing device shifted the dmax closer to the skin thereby causing an increase in the

skin dose. Taking into consideration the field size of 10 x 10 cm2, a PDD of 44.42 was

recorded for the measurements without the mask and increased to 48.61, 55.61, and 64.98

for the measurements with the 10 cm stretched mask, 5 cm stretched mask and the

unstretched mask respectively at a depth of 0.0 cm. As the field sizes increased for a

particular thermoplastic mask thickness, the skin dose also increased, with the


58

unstretched mask recording the highest followed by the 5 cm stretched mask; 10 cm

stretched mask, and no mask recording the least skin dose.

Figure 4. 12: A bar chart showing the effect of different mask thickness on PDD at depth 0.0 cm.

Figure 4.13 also illustrates a plot of skin dose at different field sizes against the various

mask stretches. The difference in skin doses for the various stretches was compared with

the measurement without the mask at the different field sizes and tabulated below in

Table 4.5. A variation within 1% was recorded although the skin dose decreased as the

mask was extended.


59

Figure 4. 13: A bar chart showing the effect of different mask thickness on the skin dose at depth 0.0 cm.

Table 4. 5: Dose (Gy) differences between the various mask stretches and without the mask

Field sizes (cm2) Without Mask Unstretched

Mask 5 cm Stretched

Mask 10 cm Stretched

Mask 5 x 5 0.39 0.21 0.13 0.05

10 x 10 0.45 0.21 0.13 0.05 15 x 15 0.51 0.21 0.13 0.05 20 x 20 0.57 0.20 0.12 0.05

The skin-sparing effect which is a significant factor of higher energy beams was achieved

when there was no mask present and also when the 10 cm stretched mask was used. On

the other hand, when the thermoplastic mask was overstretched to minimize the build-up

effect and increase the skin sparing effect, the masks rigidity was compromised thus

reducing its ability to function properly as an immobilization device. Figure 4.13 above

illustrates the increase in skin dose due to the presence of the thermoplastic mask at



60

4.4.1 Effect of Field Size on Skin Dose.

As mentioned in literature, field size is one of the factors that affect a patient’s skin dose.

Table 4.6 shows an increase in skin doses as the field size was increased for different

thermoplastic mask stretches. The increase in skin dose with increasing field size was

manifested in both measurements with and without the thermoplastic mask. The

increasing skin dose was associated with the increase in electron scattering from the air,

the collimators any other materials in the path of the radiation beam. A skin dose of 0.60

Gy was measured at a field size of 5 x 5 cm2 and later increased to 0.77 Gy at a field size

of 20 x 20 cm2 for the unstretched mask. This conforms with other results in literature

(Hadley et al. 2005; Khan, 2014; Kry, Smith et al. 2012), which stated an increase in skin

dose with increasing field size.

Table 4. 6: Skin Doses (Gy) at a depth of 0.0 cm for the various Stretches of the thermoplastic mask.

Field sizes (cm2)

Without Mask

10 cm Stretched Mask

5 cm Stretched Mask

Unstretched Mask

5X5 0.39 0.44 0.52 0.60 10X10 0.45 0.51 0.58 0.66 15X15 0.51 0.57 0.64 0.72 20X20 0.57 0.63 0.70 0.77

4.4.2 Verification of Prescribed Dose

The data obtained after the dose verification was compared to the delivered dose at both

the skin surface and the dmax. With or without the mask, the surface dose increased with

increasing field size. The skin dose increased whilst the dmax shifted closer to the skin

surface due to the “bolus” effect of the mask present. The percentage differences obtained


61

were all less than 1% which falls within the tolerance limit of ±1%. The acquired data are

presented in appendix 11.

4.5 Thermoplastic Factors, TF

The thermoplastic mask factor as stated in chapter three is expressed as a ratio of the

corrected ion chamber readings with mask I1 to that without mask I0. The values in the

Tables 4.7, 4.8 and 4.9 were obtained using equation (3.3). Tables 4.7 – 4.9 present the

thermoplastic factors obtained from the stretched and unstretched thermoplastic masks at

varying depths and field sizes. A close observation of Tables 4.7 – 4.9 clearly show that

the unstretched mask had higher thermoplastic factors at the skin surface (that is, at depth

0.0 cm) followed by the 5 cm stretched mask with the 10 cm stretched masks having the

lowest for all the different field sizes. The thicker the mask, the higher the thermoplastic

factor at the skin surface. As the depth increased beyond the skin surface within the

phantom, the thermoplastic factor of the 10 cm stretched mask increased significantly

followed by the 5 cm stretched mask with the unstretched mask having the least values.

For a particular mask type (unstretched, 5 cm stretched, or 10 cm stretched) the

thermoplastic factor decreased with increasing field size.

Table 4. 7: Thermoplastic factor for the unstretched thermoplastic mask.

Depth (cm) Field Size (cm2)

5x5 10x10 15x15 20x20

0 1.535902 1.4631 1.40447 1.35071 1.6 0.999406 1.00028 1.00027 1.00242 4 0.997445 0.99701 0.99654 0.99549 6 0.992125 0.99398 0.99634 0.99763 8 0.993163 0.99169 0.99354 0.99438

10 0.990775 0.99337 0.99493 0.99635


62

Table 4. 8: Thermoplastic factor for the 5 cm stretched thermoplastic mask.


5x5 10x10 15x15 20x20

0 1.322636 1.28119 1.24728 1.215943 1.6 1.024368 1.02342 1.02225 1.021798 4 1.026509 1.02388 1.02164 1.021408 6 1.02703 1.02473 1.02284 1.021913 8 1.028693 1.02653 1.02357 1.022167

10 1.031169 1.02832 1.02619 1.023549

Table 4. 9: Thermoplastic factor for the 10 cm stretched thermoplastic mask.


5x5 10x10 15x15 20x20

0 1.138357 1.121634 1.107547 1.094308 1.6 1.025557 1.025113 1.025 1.025565 4 1.030661 1.027463 1.023947 1.023099

6 1.029334 1.028218 1.0265 1.026355

8 1.03299 1.029242 1.026154 1.023416

10 1.032063 1.029488 1.02733 1.026202

4.6 Calibration Curve

Table 4.10 gives the measured net optical density (Net OD) as a function of absorbed

dose (Gy) for the calibrated GafChromic EBT3 films. These optical densities were

obtained from equation (3.4) in chapter three. Figure 4.14 also illustrates a plot of

calculated net optical densities against the known absorbed doses for the calibrated

GafChromic EBT3 film.


63

Table 4. 10: Net Optical Densities as a function of absorbed dose for the calibrated GafChromic films Absorbed Dose

(Gy) Red Green Blue

0 0 0 0 0.20 0.04376 0.016714 0.00103 0.40 0.07846 0.032179 0.00465 0.80 0.13711 0.066814 0.01564 1.60 0.22021 0.127031 0.03478 2.40 0.2696 0.177514 0.05165 3.20 0.3015 0.220504 0.06437 4.00 0.32749 0.254787 0.07766 5.00 0.35221 0.301327 0.09495

Figure 4. 14: Calibration curve comparing RGB channels for 6 MV photon energy.

A close observation of figure 4.14 illustrates the calibration curve for the RGB channels

using a 6 MV energy where the red channel is said to give a better dose variation and

higher sensitivity as compared to the green and blue channels. From (Borca et al., 2013b),


64

the red channel had a larger response up to 10 Gy with the green channel exceeding the

red one for radiation doses above 10 Gy. On the other hand, the blue channel had a lower

response gradient at any dose because the signal has a weak dose dependence while

having a strong dependence on the thickness of the active layer. Thus, this makes the blue

channel less suitable than the other channels for dose measurements. Consequently,

further analysis was done on the red channels. Figure 4.15 shows a plot of absorbed dose

against net optical density for the selected red channel.

Figure 4. 15: A graph of Absorbed Dose against Net Optical Density for the Red channel.

The corresponding calibration equation obtained from figure 4.15 for the conversion of

the other film responses to dose is given by equation (4)

y = a + bx + cx2 + dx3 + ex4 + … ……… (4)

where a = -3.8656, b = 813.23, c = -3958.1, d = 16105 and the regression R2 = 0.9994.


65

4.6.1 Dose Determination for the Irradiated GafChromic EBT3 Films.

4.6.1.1 Net Optical Density (Net OD)

Tables 4.11 – 4.14 illustrate the Net Optical Densities (Net OD) obtained after the

scanning and reading of the GafChromic films which were irradiated with the unknown

radiation doses.

Table 4. 11: Net Optical Densities (Net OD) obtained for the GafChromic films without the mask. Depth

(cm)

Field size (cm2)

5 x 5 10 x 10 15 x 15 20 x 20

0.0 0.03844 0.05005 0.07276 0.07655 1.6 0.17499 0.1871 0.18661 0.18521

10.0 0.12877 0.13628 0.14948 0.14728

Table 4. 12: Net Optical Densities (Net OD) obtained for the GafChromic films with the unstretched mask.

Depth

(cm)

Field size (cm2)

5 x 5 10 x 10 15 x 15 20 x 20

0.0 0.11937 0.12846 0.131 0.14826

1.6 0.18217 0.19709 0.18861 0.1962

10.0 0.13031 0.14469 0.13631 0.14736

Table 4. 13: Net Optical Densities (Net OD) obtained for the GafChromic films with the 5 cm stretched mask.

Depth

(cm)

Field size (cm2)

5 x 5 10 x 10 15 x 15 20 x 20

0.0 0.08675 0.10143 0.10836 0.10821

1.6 0.17619 0.18442 0.19683 0.18637

10.0 0.12888 0.13613 0.15135 0.13732


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Table 4. 14: Net Optical Densities (Net OD) obtained for the GafChromic films with the 10 cm stretched mask.

Depth

(cm)

Field size (cm2)

5 x 5 10 x 10 15 x 15 20 x 20

0.0 0.08193 0.06123 0.09666 0.09611 1.6 0.19743 0.18116 0.19317 0.18977

10.0 0.14362 0.13692 0.15554 0.15079

4.6.1.2 GafChromic Film Doses

The above-tabulated net optical densities with and without the thermoplastic mask for the

various depths and field sizes were used to determine the doses deposited in the

GafChromic films. These were done by using equation (4.1) from the calibration curve of

the absorbed dose against the net optical density:

y = 16105x3 - 3958.1x2 + 813.23x - 3.8656 ……… (4.1)

Tables 4.15 – 4.18 represent the calculated absorbed doses acquired from the

GafChromic films using equation (4.1).

Table 4. 15: Calculated absorbed dose (Gy) values for various field sizes at different depths without the mask.

Depth (cm) Field size (cm2)

5 x 5 10 x 10 15 x 15 20 x 20

0 0.22 0.29 0.41 0.42

1.6 1.04 1.15 1.15 1.13

10 0.70 0.74 0.83 0.82

A close observation of Table 4.15 shows that in the absence of the thermoplastic mask for

the field size of 10 x 10 cm2, the surface dose elevated from 0.29 Gy to 1.15 Gy at dmax

(1.6 cm) and later dropped to 0.74 Gy at the reference depth of 10 cm. Taking into

consideration the skin dose, dose at maximum depth, and dose at a reference depth of 10


67

cm, it can be seen that with increasing field size from 5 x 5 to 20 x 20 cm2 the skin dose

increased from 0.22 Gy to 0.42 Gy; dmax also increased from 1.04 Gy of 5 x 5 field size to

1.15Gy at a field size of 10 x 10 and later dropped to 1.13 Gy at 20 x 20 field size; and at

the reference depth of 10 cm the dose increased from 0.70 Gy to 0.83Gy of field size of

15 x15 and also dropped to 0.82 Gy at a field size of 20 x 20 respectively. For the various

field sizes, the skin dose increased by a factor of 0.06% (for 5 x 5 to 10 x 10 cm2), 0.11%

(for 10 x 10 to 15 x 15 cm2), and 0.02% (for 5 x 5 to 10 x 10 cm2). Thus, these results

conform with literature on the aspect of increasing depth for a particular field size but

deviate from literature in the aspect of increasing field sizes for dmax (1.6 cm) and a depth

of 10 cm as indicated in (F. M. Khan, 2014).

Table 4. 16: Calculated absorbed dose (Gy) values for various field sizes at different depths with the unstretched mask.


5 x 5 10 x 10 15 x 15 20 x 20

0 0.64 0.69 0.71 0.82

1.6 1.10 1.26 1.17 1.25

10 0.71 0.80 0.74 0.82

A close observation of Table 4.16 shows that the presence of the unstretched mask for a

field size of 10 x 10 cm2, the surface dose increased from 0.29 Gy (without the mask) to

0.69 Gy by a factor of 0.38%. The rise in skin dose was ascribed to the “bolus” effect due

to the presence of the mask. However, comparing the dose at maximum depth and at the

reference depth of 10 cm without a mask with the unstretched mask, it can be said that

there was an insignificant change in the dose values. Also, the skin dose for the

unstretched mask raised from 0.69 Gy to 1.26 Gy at dmax (1.6 cm) and later dropped to

0.80 Gy at the depth of 10 cm. Taking into account the skin dose, dose at maximum


68

depth, and dose at a reference depth of 10 cm, it can be seen that with increasing field

size from 5 x 5 to 20 x 20 cm2 the skin dose increased from 0.64 Gy to 0.82 Gy; dmax had

fluctuations within the various field sizes; and at the depth of 10 cm the dose also

fluctuated within the field sizes respectively. For the various field sizes, the skin dose

increased by a factor of 0.06% (for 5 x 5 to 10 x 10 cm2), 0.11% (for 10 x 10 to 15 x 15

cm2), and 0.02% (for 5 x 5 to 10 x 10 cm2). Thus, these results conform with literature on

the aspect of increasing depth for a particular field size but deviate from literature in the

aspect of increasing field sizes for dmax (1.6 cm) and a depth of 10 cm as indicated

in(Hadley et al., 2005; F. M. Khan, 2014; Kry et al., 2012).

Table 4. 17: Calculated absorbed dose (Gy) values for various field sizes at different depths with the 5 cm stretched mask.


5x5 10x10 15x15 20x20

0 0.47 0.55 0.58 0.58

1.6 1.05 1.13 1.26 1.14

10 0.70 0.74 0.84 0.75

A close observation of Table 4.17 shows that with the presence of the 5 cm stretched

mask for a field size of 10 x 10 cm2, the surface dose decreased from 0.69 Gy (with the

unstretched mask) to 0.55 Gy by a factor of 0.17%. The decrease in skin dose was as well

attributed to the reduction in the thickness of the mask as a result of the slight stretching.

However, comparing the dose at maximum depth and at the reference depth of 10 cm of

the unstretched mask with the 5 cm stretched one, it can be said that there was an

insignificant change in the dose values by factors of 0.05% and 0.008%. Considering the

skin dose, dose at maximum depth, and dose at a reference depth of 10 cm, while

increasing field size from 5 x 5 to 20 x 20 cm2, the skin dose increased from 0.47 Gy to


69

0.58 Gy; dmax had fluctuations within the different field sizes; and at the depth of 10 cm

the dose also fluctuated within the field sizes respectively. For the various field sizes, the

skin dose increased by a factor of 0.07% (for 5 x 5 to 10 x 10 cm2), 0.04% (for 10 x 10 to

15 x 15 cm2), and 0.0008% (for 5 x 5 to 10 x 10 cm2). Thus, these results conform with

literature on the aspect of increasing depth for a particular field size but deviate from

literature in the aspect of increasing field sizes for dmax (1.6 cm) and a depth of 10 cm as

indicated in (Hadley et al., 2005; F. M. Khan, 2014; Kry et al., 2012).

Table 4. 18: Calculated absorbed dose (Gy) values for various field sizes at different depths with the 10 cm stretched mask.


5x5 10x10 15x15 20x20

0 0.45 0.35 0.52 0.52

1.6 1.26 1.09 1.22 1.18

10 0.79 0.75 0.87 0.84

From the observation of Table 4.18, it shows that with the presence of the 10 cm

stretched mask for a field size of 10 x 10 cm2, the surface dose decreased from 0.55 Gy

(with the 5 cm stretched mask) to 0.35 Gy by a factor of 0.086%. This decrease in the

skin dose was as well attributed to the reduction in the thickness of the mask as a result of

the full stretching. However, comparing the dose at maximum depth and at a reference

depth of 10 cm of the 5 cm stretched mask with the 10 cm stretched one, it can be

deduced that there was an insignificant change in the dose values by factors of 0.05% and

0.06%. Considering the skin dose, dose at maximum depth, and dose at a reference depth

of 10 cm, while increasing field size from 5 x 5 to 20 x 20 cm2, the skin dose fluctuated

between 0.35 Gy and 0.52 Gy; dmax had fluctuations within the different field sizes; and at


70

the depth of 10 cm the dose also fluctuated within the field sizes respectively. Thus, these

results conform with literature on the aspect of increasing depth for a particular field size

but deviate from literature in the aspect of increasing field sizes for a particular depth as

indicated in (Hadley et al., 2005; F. M. Khan, 2014; Kry et al., 2012).

4.6.1.2.1 Graphical Representations of Absorbed dose against Depth for the

GafChromic film.

Figure 4. 16: A graph showing the variation of absorbed dose versus depth and field size for GafChromic

measurements without the mask.


71


measurements with the unstretched mask.


measurements with the 5 cm stretched mask.


72


measurements with the 10 cm stretched mask.


73

CHAPTER FIVE

CONCLUSION AND RECOMMENDATIONS

5.1 Conclusions

The purpose of this research was to examine and analyze the effects of thermoplastic

masks used as immobilization devices for cancer treatments in the head and neck region

on the skin and the build-up region doses in terms of the thermoplastic percentage depth

dose, thermoplastic factor and the changes in the skin dose and build-up region.

Taking into consideration measurements done with the ionization chamber, the surface

dose, the dose at maximum depth (dmax) and the dose at a reference depth of 10 cm were

found to increase with increasing mask thickness and increasing field size respectively.

Due to the presence of the thermoplastic mask during measurements, the dose in the

build-up region increased, subsequently shifting dmax, to increasingly lower depths to the

skin surface. For a field size of 10 x 10 cm, an increase in skin dose of 0.21, 0.13 and

0.05 Gy was estimated for the unstretched mask, the 5 cm stretched mask and the 10 cm

stretched mask respectively, which was ascribed to the “bolus” effect of the thermoplastic

masks. For the various field sizes, the skin dose increased by an average factor less than

1% for the different mask stretches. However, a variation within 1% was recorded

although the skin dose decreased as the mask was extended.

The thermoplastic factor at the skin surface was also found to increase with an increasing

mask thickness and for a particular mask type (unstretched, 5 cm stretched, or 10 cm

stretched) the thermoplastic factor decreased with increasing field size. With respect to

the doses obtained using the GafChromic EBT3 films, there were lots of discrepancies in


74

the values which varied significantly with those obtained using the ionization chamber.

The main cause behind the variation between the GafChromic EBT3 film and the

ionization chamber dose values is the inhomogeneous composition of the GafChromic

film emanating from the film’s manufacturing process. Note that the film and the ion

chamber measurements were yielding and comparing absolute dose values, that is, no

normalization was applied.

It can generally be concluded that multiple factors contributed to the observed increase in

skin dose during the measurements and these include, the bolus effect of the

immobilization mask (the thickness of the mask use), the inadvertent inclusion of the skin

as a part of the target volume and field size.

The results obtained from the dose verification using the anthropomorphic phantom and

diodes clearly confirms the fact that the presence of the thermoplastic mask and

increasing field size with or without the mask increases the skin dose. The percentage

differences obtained were all less than 1% which falls within the tolerance limit of ±1%.

A vivid comparison of this work with that performed by (Adu-poku, 2017) a formal

student of the Department of Medical Physics, on the “Dosimetric Effects of

Thermoplastic Immobilizing Devices on Skin Dose” was done. It was found that the skin

dose, the dose at maximum depth and at a reference depth of 10 cm, increased with

increasing mask thickness and also increasing the field sizes respectively. With respect to

the thermoplastic mask factor, both studies found that it increased at the skin surface as

the mask thickness was increased and for a particular mask type (stretched or

unstretched) the thermoplastic mask factor decreased with increasing field size. In both

situations, the rise in dose was associated with the “bolus” effect caused by the presence


75

of the thermoplastic mask. Although, her work was performed on a Cobalt-60 (energy =

1.25 MeV) machine at Komfo Anokye Teaching Hospital, Kumasi, a variation within 1%

was recorded although the skin dose decreased as the mask was extended for both

studies.

5.2 Recommendations

At the Sweden Ghana Medical Center, when a thermoplastic mask is to be used at

simulation, it should be stretched firmly toward the patient’s skin surface to thin the

polystyrene in the targeted area in order to minimize the bolus effect, the build-up effect

and increase the skin sparing effect, bearing in mind not to compromise the rigidity of the

mask as it will decrease the mask’s capability to function properly as an immobilization

device. Also, taking into account the skin as one of the sensitive structures will be

possible to reduce the skin dose to a tolerable level without compromising the tumor

target coverage during the treatment planning.

Since thermoplastic immobilization masks are said to increase the dose to the skin

significantly, it is recommended that the thermoplastic mask factor should be

incorporated into the treatment planning system, just as in tray factors, in order to account

for the effect of the increase in skin dose.

5.3 Further Research

Additional work should be done using two different beam energies (example 6 MV and

15 MV) to study and compare the effect of the thermoplastic mask used as immobilizing

device on skin dose.


76

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82

APPENDICES

APPENDIX 1

Data showing the physical thicknesses of the various thermoplastic masks with its

corresponding hole dimensions.

APPENDIX 1A: Physical mask thicknesses with corresponding hole dimensions.

Mask thickness (mm) Hole diameter (mm)

Unstretched mask 2.30 -

5 cm stretched mask 1.20 2.30

10 cm stretched mask 0.86 5.25

APPENDIX 2

Raw data showing average electrometer readings (nC/min) with standard deviations on

the solid water phantom measurements with and without the thermoplastic mask at


APPENDIX 2A: Electrometer readings without the mask.

Depth

(cm)

Field Size (cm2)

5x5 10x10 15x15 20x20

0 4.51 ± 0.00 5.25 ± 0.00 5.92 ± 0.00 6.62 ± 0.00 1.6 11.22 ± 0.01 11.81 ± 0.01 12.13 ± 0.01 12.39 ± 0.01 4 10.44 ± 0.01 11.17 ± 0.01 11.55 ± 0.01 11.83 ± 0.01 6 9.69 ± 0.00 10.51 ± 0.01 10.94 ± 0.01 11.26 ± 0.01 8 8.92 ± 0.00 9.83 ± 0.00 10.32 ± 0.01 10.68 ± 0.01

10 8.20 ± 0.00 9.15 ± 0.00 9.67 ± 0.00 10.05 ± 0.00


83

APPENDIX 2B: Electrometer readings with the unstretched mask.

Depth

(cm)

Field Size (cm2)

5x5 10x10 15x15 20x20

0 6.92 ± 0.01 7.68 ± 0.00 8.32 ± 0.00 8.95 ± 0.00 1.6 11.21 ± 0.01 11.82 ± 0.02 12.14 ± 0.01 12.42 ± 0.01 4 10.41 ± 0.01 11.13 ± 0.01 11.51 ± 0.01 11.78 ± 0.01 6 9.62 ± 0.00 10.45 ± 0.01 10.90 ± 0.01 11.23 ± 0.01 8 8.86 ± 0.00 9.74 ± 0.00 10.26 ± 0.01 10.62 ± 0.01

10 8.13 ± 0.00 9.08 ± 0.00 9.62 ± 0.00 10.01 ± 0.01

APPENDIX 2C: Electrometer readings with the 5 cm stretched mask.

Depth

(cm)

Field Size (cm2)

5x5 10x10 15x15 20x20

0 5.96 ± 0.00 6.72 ± 0.00 7.39 ± 0.00 8.05 ± 0.00 1.6 11.49 ± 0.00 12.09 ± 0.00 12.40 ± 0.01 12.66 ± 0.01 4 10.71 ± 0.01 11.43 ± 0.01 11.80 ± 0.01 12.09 ± 0.01 6 9.96 ± 0.00 10.77 ± 0.01 11.19 ± 0.01 11.50 ± 0.01 8 9.18 ± 0.00 10.09 ± 0.01 10.57 ± 0.01 10.91 ± 0.01

10 8.46 ± 0.00 9.40 ± 0.00 9.93 ± 0.00 10.29 ± 0.01

APPENDIX 2D: Electrometer readings with the 10 cm stretched mask.

Depth

(cm)

Field Size (cm2)

5x5 10x10 15x15 20x20

0 5.13 ± 0.00 5.89 ± 0.00 6.56 ± 0.00 7.25 ± 0.00 1.6 11.50 ± 0.01 12.11 ± 0.00 12.44 ± 0.01 12.70 ± 0.01 4 10.76 ± 0.01 11.47 ± 0.01 11.83 ± 0.00 12.11 ± 0.01 6 9.98 ± 0.00 10.81 ± 0.01 11.23 ± 0.01 11.55 ± 0.01 8 9.22 ± 0.00 10.11 ± 0.01 10.59 ± 0.01 10.93 ± 0.01

10 8.47 ± 0.00 9.41 ± 0.00 9.94 ± 0.00 10.31 ± 0.01


84

APPENDIX 3

Percentage Depth Dose (PDD) values for the ion chamber with and without the

thermoplastic mask at various field sizes.

APPENDIX 3A: PDD values without the mask.

Depth

(cm)

Field Size (cm2)

5x5 10x10 15x15 20x20

0 40.19 44.42 48.82 53.47

1.6 100.00 100.00 100.00 100.00

4 93.05 94.53 95.22 95.53

6 86.42 89.00 90.19 90.88

8 79.54 83.18 85.08 86.19

10 73.13 77.41 79.71 81.14

APPENDIX 3B: PDD values with the unstretched mask.

Depth

(cm)

Field Size (cm2)

5x5 10x10 15x15 20x20

0 61.76 64.98 68.54 72.05

1.6 100.00 100.00 100.00 100.00

4 92.86 94.22 94.86 94.87

6 85.79 88.43 89.84 90.44

8 79.05 82.46 84.51 85.50

10 72.50 76.88 79.29 80.64


85

APPENDIX 3C: PDD values with the 5 cm stretched mask.

Depth

(cm)

Field Size (cm2)

5x5 10x10 15x15 20x20

0 51.89 55.61 59.56 63.63

1.6 100.00 100.00 100.00 100.00

4 93.24 94.57 95.16 95.50

6 86.64 89.11 90.24 90.89

8 79.88 83.43 85.19 86.23

10 73.61 77.78 80.02 81.27

APPENDIX 3D: PDD values with the 10 cm stretched mask.

Depth

(cm)

Field Size (cm2)

5x5 10x10 15x15 20x20

0 44.61 48.61 52.75 57.06

1.6 100.00 100.00 100.00 100.00

4 93.51 94.74 95.12 95.30

6 86.73 89.27 90.32 90.95

8 80.12 83.51 85.18 86.01

10 73.59 77.74 79.90 81.19


86

APPENDIX 4

Percentage Depth Dose (PDD) values at a depth of 0.0 cm for the various field sizes and

mask stretches.

APPENDIX 4A: PDD values at the skin surface for the various field sizes and mask stretches.

Field sizes (cm2) Without Mask

Unstretched Mask

5 cm Stretched Mask

10 cm Stretched Mask

5X5 40.19 61.76 51.89 44.61

10X10 44.42 64.98 55.61 48.61

15X15 48.82 68.54 59.56 52.75

20X20 53.47 72.05 63.63 57.06

APPENDIX 5

Percentage Depth Dose (PDD) values against Depth for the various field sizes.

APPENDIX 5A: Field size of 5 x 5 cm2

Depth (cm) Without Mask Unstretched

Mask 5 cm Stretched


Mask

0 40.19 61.76 51.89 44.61

1.6 100.00 100.00 100.00 100.00

4 93.05 92.86 93.24 93.51

6 86.42 85.79 86.64 86.73

8 79.54 79.05 79.88 80.12

10 73.13 72.50 73.61 73.59


87

APPENDIX 5B: Field size of 10 x 10 cm2


Mask 5 cm Stretched


Mask

0 44.42 64.98 55.61 48.61

1.6 100.00 100.00 100.00 100.00

4 94.53 94.22 94.57 94.74

6 89.00 88.43 89.11 89.27

8 83.18 82.46 83.43 83.51

10 77.41 76.88 77.78 77.74

APPENDIX 5C: Field size of 15 x 15 cm2


Mask 5 cm Stretched


Mask

0 48.82 68.54 59.56 52.75

1.6 100.00 100.00 100.00 100.00

4 95.22 94.86 95.16 95.12

6 90.19 89.84 90.24 90.32

8 85.08 84.51 85.19 85.18

10 79.71 79.29 80.02 79.90

APPENDIX 5D: Field size of 20 x 20 cm2


Mask 5 cm Stretched


Mask

0 53.47 72.05 63.63 57.06

1.6 100.00 100.00 100.00 100.00

4 95.53 94.87 95.50 95.30

6 90.88 90.44 90.89 90.95

8 86.19 85.50 86.23 86.01

10 81.14 80.64 81.27 81.19


88

APPENDIX 6

Raw RGB data obtained after scanning and reading of the GafChromic EBT3 films for

calibration.

APPENDIX 6A: Data from the red channel of the GafChromic EBT3 film

Label Area Mean StdDev Min Max

1 Cal001.tif (red):0086-0283 1600 94.611 0.826 93 98

2 Cal001.tif (red):0170-0281 1600 85.543 0.973 83 89

3 Cal001.tif (red):0252-0281 1600 78.973 0.831 75 81

4 Cal001.tif (red):0335-0281 1600 68.998 0.706 66 72

5 Cal001.tif (red):0414-0284 1600 56.981 0.76 55 62

6 Cal001.tif (red):0493-0282 1600 50.856 0.996 48 61

7 Cal001.tif (red):0577-0282 1600 47.254 0.938 45 64

8 Cal001.tif (red):0656-0282 1600 44.509 0.798 42 48

9 Cal001.tif (red):0742-0279 1600 42.047 0.958 38 46

APPENDIX 6B: Data from the green channel of the GafChromic EBT3 film


1 Cal001.tif (green):0086-0284 1600 128.862 0.907 126 133

2 Cal001.tif (green):0169-0282 1600 123.997 1.332 119 128

3 Cal001.tif (green):0250-0281 1600 119.659 1.021 115 123

4 Cal001.tif (green):0335-0279 1600 110.487 1.067 96 114

5 Cal001.tif (green):0413-0281 1600 96.182 0.92 94 99

6 Cal001.tif (green):0494-0282 1600 85.627 1.058 83 89

7 Cal001.tif (green):0574-0282 1600 77.557 0.882 74 84

8 Cal001.tif (green):0656-0281 1600 71.67 0.886 69 75

9 Cal001.tif (green):0742-0281 1600 64.387 0.758 62 67

APPENDIX 6C: Data from the blue channel of the GafChromic EBT3 film


1 Cal001.tif (blue):0086-0282 1600 71.726 0.803 69 76

2 Cal001.tif (blue):0170-0281 1600 71.556 0.906 69 75

3 Cal001.tif (blue):0249-0282 1600 70.962 0.836 68 75

4 Cal001.tif (blue):0334-0282 1600 69.189 0.813 62 72

5 Cal001.tif (blue):0413-0284 1600 66.206 0.798 64 70

6 Cal001.tif (blue):0493-0281 1600 63.684 1.152 60 74

7 Cal001.tif (blue):0575-0282 1600 61.845 0.881 60 66

8 Cal001.tif (blue):0654-0283 1600 59.981 0.839 58 64

9 Cal001.tif (blue):0740-0279 1600 57.641 0.785 55 60


89

APPENDIX 7

Raw data of the red channel obtained from the films irradiated with unknown doses for

the various field sizes without the thermoplastic mask.



Unexposed GWM 5x5001.tif (red):0155-0291 1600 93.039 0.987 90 97

A GWM 5x5001.tif (red):0306-0275 1600 85.158 0.917 79 87

B GWM 5x5001.tif (red):0457-0276 1600 62.183 2.063 30 73

C GWM 5x5001.tif (red):0615-0276 1600 69.166 1.211 59 74




A GWM 10x10002.tif (red):0278-0286 1600 82.389 0.825 79 85

B GWM 10x10002.tif (red):0423-0286 1600 60.093 0.975 58 79

C GWM 10x10002.tif (red):0574-0282 1600 67.553 0.97 64 76




A GWM 15x15003.tif (red):0288-0281 1600 79.484 0.921 77 82

B GWM 15x15003.tif (red):0457-0285 1600 61.156 0.845 59 68

C GWM 15x15003.tif (red):0615-0283 1600 66.614 0.832 64 74




A GWM 20x20004.tif (red):0291-0285 1600 77.068 0.916 73 80

B GWM 20x20004.tif (red):0449-0279 1600 60.008 1.61 34 78

C GWM 20x20004.tif (red):0600-0288 1600 65.484 1.068 60 73


90

APPENDIX 8


the various field sizes with the unstretched thermoplastic mask.



Unexposed GUM 5x5001.tif (red):0136-0301 1600 92.969 1.023 88 97

A GUM 5x5001.tif (red):0282-0286 1600 70.627 2.48 62 76

B GUM 5x5001.tif (red):0436-0288 1600 61.118 0.911 58 71

C GUM 5x5001.tif (red):0578-0289 1600 68.869 1.734 62 73




A GUM 10x10002.tif (red):0266-0280 1600 70.021 1.701 66 75

B GUM 10x10002.tif (red):0408-0285 1600 59.786 0.862 56 62

C GUM 10x10002.tif (red):0552-0283 1600 67.452 0.928 64 71




A GUM 15x15003.tif (red):0279-0283 1600 68.656 1.17 65 73

B GUM 15x15003.tif (red):0432-0289 1600 60.127 0.838 58 69

C GUM 15x15003.tif (red):0578-0282 1600 67.821 0.927 65 75




A GUM 20x20004.tif (red):0279-0286 1600 66.856 1.043 63 70

B GUM 20x20004.tif (red):0429-0284 1600 59.869 1.046 53 77

C GUM 20x20004.tif (red):0572-0284 1600 66.994 0.915 64 70


91

APPENDIX 9


the various field sizes with the 5 cm stretched thermoplastic mask.



Unexposed GSS 5x5001.tif (red):0120-0293 1600 91.639 0.971 89 94

A GSS 5x5001.tif (red):0253-0289 1600 75.046 2.152 71 82

B GSS 5x5001.tif (red):0392-0286 1600 61.079 0.763 58 69

C GSS 5x5001.tif (red):0527-0286 1600 68.109 0.779 66 71




A GSS 10x10002.tif (red):0265-0276 1600 72.863 1.498 69 78

B GSS 10x10002.tif (red):0405-0284 1600 60.189 0.838 58 63

C GSS 10x10002.tif (red):0544-0276 1600 67.268 0.981 58 71




A GSS 15x15003.tif (red):0268-0283 1600 72.993 1.416 69 78

B GSS 15x15003.tif (red):0418-0285 1600 59.54 0.873 55 70

C GSS 15x15003.tif (red):0564-0281 1600 66.114 0.95 62 69




A GSS 20x20004.tif (red):0281-0283 1600 71.17 1.668 66 76

B GSS 20x20004.tif (red):0438-0284 1600 59.448 1.04 57 77

C GSS 20x20004.tif (red):0589-0284 1600 66.555 0.966 60 73


92

APPENDIX 10


the various field sizes with the 10 cm stretched thermoplastic mask.



Unexposed GFS 5x5001.tif (red):0128-0289 1600 96.456 0.941 93 99

A GFS 5x5001.tif (red):0252-0283 1600 79.873 2.585 75 86

B GFS 5x5001.tif (red):0375-0280 1600 61.221 0.97 59 74

C GFS 5x5001.tif (red):0495-0279 1600 69.296 0.895 67 73




A GFS 10x10002.tif (red):0245-0270 1600 79.478 2.357 73 84

B GFS 10x10002.tif (red):0372-0270 1600 60.299 0.853 57 63

C GFS 10x10002.tif (red):0500-0270 1600 66.765 0.936 64 70




A GFS 15x15003.tif (red):0261-0266 1600 75.044 2.125 70 80

B GFS 15x15003.tif (red):0396-0271 1600 60.09 0.805 58 63

C GFS 15x15003.tif (red):0532-0264 1600 65.53 1.079 60 75




A GFS 20x20004.tif (red):0262-0279 1600 74.578 1.836 63 79

B GFS 20x20004.tif (red):0412-0283 1600 60.111 0.734 58 63

C GFS 20x20004.tif (red):0540-0275 1600 65.756 0.912 61 71


93

APPENDIX 11

Data from dose verification in the head and neck region using the anthropomorphic

phantom for a 6 MV photon energy.

APPENDIX 11A: Dose verification without the mask at the skin surface

Field size (cm2)

Planned dose at d = 0.0 cm

Delivered dose at d = 0.0 cm

% Dev. Tolerance

10 x 10 1.10 1.09 0.91 ±1

20 x 20 1.12 1.11 0.89 ±1

APPENDIX 11B: Dose verification with the mask at skin surface

Field size (cm2)



% Dev. Tolerance

10 x 10 1.19 1.18 0.84 ±1

20 x 20 1.23 1.22 0.81 ±1

APPENDIX 11C: Dose verification without the mask at dmax

Field size (cm2)



% Dev. Tolerance

10 x 10 1.38 1.37 0.72 ±1

20 x 20 1.44 1.43 0.69 ±1

APPENDIX 11D: Dose verification with the mask at dmax

Field size (cm2)



% Dev. Tolerance

10 x 10 1.39 1.38 0.72 ±1

20 x 20 1.45 1.45 0 ±1
